2563c97f61
Reviewed-by: Juan Quintela <quintela@redhat.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org> Signed-off-by: Richard Henderson <richard.henderson@linaro.org> Message-Id: <20231221031652.119827-72-richard.henderson@linaro.org>
1515 lines
59 KiB
ReStructuredText
1515 lines
59 KiB
ReStructuredText
=========
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Migration
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=========
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QEMU has code to load/save the state of the guest that it is running.
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These are two complementary operations. Saving the state just does
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that, saves the state for each device that the guest is running.
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Restoring a guest is just the opposite operation: we need to load the
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state of each device.
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For this to work, QEMU has to be launched with the same arguments the
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two times. I.e. it can only restore the state in one guest that has
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the same devices that the one it was saved (this last requirement can
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be relaxed a bit, but for now we can consider that configuration has
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to be exactly the same).
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Once that we are able to save/restore a guest, a new functionality is
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requested: migration. This means that QEMU is able to start in one
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machine and being "migrated" to another machine. I.e. being moved to
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another machine.
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Next was the "live migration" functionality. This is important
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because some guests run with a lot of state (specially RAM), and it
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can take a while to move all state from one machine to another. Live
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migration allows the guest to continue running while the state is
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transferred. Only while the last part of the state is transferred has
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the guest to be stopped. Typically the time that the guest is
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unresponsive during live migration is the low hundred of milliseconds
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(notice that this depends on a lot of things).
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.. contents::
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Transports
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==========
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The migration stream is normally just a byte stream that can be passed
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over any transport.
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- tcp migration: do the migration using tcp sockets
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- unix migration: do the migration using unix sockets
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- exec migration: do the migration using the stdin/stdout through a process.
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- fd migration: do the migration using a file descriptor that is
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passed to QEMU. QEMU doesn't care how this file descriptor is opened.
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In addition, support is included for migration using RDMA, which
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transports the page data using ``RDMA``, where the hardware takes care of
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transporting the pages, and the load on the CPU is much lower. While the
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internals of RDMA migration are a bit different, this isn't really visible
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outside the RAM migration code.
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All these migration protocols use the same infrastructure to
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save/restore state devices. This infrastructure is shared with the
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savevm/loadvm functionality.
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Debugging
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=========
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The migration stream can be analyzed thanks to ``scripts/analyze-migration.py``.
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Example usage:
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.. code-block:: shell
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$ qemu-system-x86_64 -display none -monitor stdio
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(qemu) migrate "exec:cat > mig"
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(qemu) q
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$ ./scripts/analyze-migration.py -f mig
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{
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"ram (3)": {
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"section sizes": {
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"pc.ram": "0x0000000008000000",
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...
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See also ``analyze-migration.py -h`` help for more options.
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Common infrastructure
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=====================
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The files, sockets or fd's that carry the migration stream are abstracted by
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the ``QEMUFile`` type (see ``migration/qemu-file.h``). In most cases this
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is connected to a subtype of ``QIOChannel`` (see ``io/``).
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Saving the state of one device
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==============================
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For most devices, the state is saved in a single call to the migration
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infrastructure; these are *non-iterative* devices. The data for these
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devices is sent at the end of precopy migration, when the CPUs are paused.
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There are also *iterative* devices, which contain a very large amount of
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data (e.g. RAM or large tables). See the iterative device section below.
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General advice for device developers
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------------------------------------
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- The migration state saved should reflect the device being modelled rather
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than the way your implementation works. That way if you change the implementation
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later the migration stream will stay compatible. That model may include
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internal state that's not directly visible in a register.
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- When saving a migration stream the device code may walk and check
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the state of the device. These checks might fail in various ways (e.g.
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discovering internal state is corrupt or that the guest has done something bad).
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Consider carefully before asserting/aborting at this point, since the
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normal response from users is that *migration broke their VM* since it had
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apparently been running fine until then. In these error cases, the device
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should log a message indicating the cause of error, and should consider
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putting the device into an error state, allowing the rest of the VM to
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continue execution.
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- The migration might happen at an inconvenient point,
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e.g. right in the middle of the guest reprogramming the device, during
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guest reboot or shutdown or while the device is waiting for external IO.
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It's strongly preferred that migrations do not fail in this situation,
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since in the cloud environment migrations might happen automatically to
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VMs that the administrator doesn't directly control.
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- If you do need to fail a migration, ensure that sufficient information
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is logged to identify what went wrong.
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- The destination should treat an incoming migration stream as hostile
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(which we do to varying degrees in the existing code). Check that offsets
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into buffers and the like can't cause overruns. Fail the incoming migration
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in the case of a corrupted stream like this.
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- Take care with internal device state or behaviour that might become
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migration version dependent. For example, the order of PCI capabilities
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is required to stay constant across migration. Another example would
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be that a special case handled by subsections (see below) might become
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much more common if a default behaviour is changed.
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- The state of the source should not be changed or destroyed by the
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outgoing migration. Migrations timing out or being failed by
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higher levels of management, or failures of the destination host are
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not unusual, and in that case the VM is restarted on the source.
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Note that the management layer can validly revert the migration
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even though the QEMU level of migration has succeeded as long as it
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does it before starting execution on the destination.
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- Buses and devices should be able to explicitly specify addresses when
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instantiated, and management tools should use those. For example,
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when hot adding USB devices it's important to specify the ports
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and addresses, since implicit ordering based on the command line order
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may be different on the destination. This can result in the
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device state being loaded into the wrong device.
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VMState
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-------
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Most device data can be described using the ``VMSTATE`` macros (mostly defined
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in ``include/migration/vmstate.h``).
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An example (from hw/input/pckbd.c)
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.. code:: c
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static const VMStateDescription vmstate_kbd = {
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.name = "pckbd",
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.version_id = 3,
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.minimum_version_id = 3,
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.fields = (const VMStateField[]) {
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VMSTATE_UINT8(write_cmd, KBDState),
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VMSTATE_UINT8(status, KBDState),
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VMSTATE_UINT8(mode, KBDState),
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VMSTATE_UINT8(pending, KBDState),
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VMSTATE_END_OF_LIST()
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}
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};
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We are declaring the state with name "pckbd". The ``version_id`` is
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3, and there are 4 uint8_t fields in the KBDState structure. We
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registered this ``VMSTATEDescription`` with one of the following
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functions. The first one will generate a device ``instance_id``
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different for each registration. Use the second one if you already
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have an id that is different for each instance of the device:
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.. code:: c
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vmstate_register_any(NULL, &vmstate_kbd, s);
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vmstate_register(NULL, instance_id, &vmstate_kbd, s);
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For devices that are ``qdev`` based, we can register the device in the class
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init function:
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.. code:: c
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dc->vmsd = &vmstate_kbd_isa;
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The VMState macros take care of ensuring that the device data section
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is formatted portably (normally big endian) and make some compile time checks
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against the types of the fields in the structures.
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VMState macros can include other VMStateDescriptions to store substructures
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(see ``VMSTATE_STRUCT_``), arrays (``VMSTATE_ARRAY_``) and variable length
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arrays (``VMSTATE_VARRAY_``). Various other macros exist for special
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cases.
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Note that the format on the wire is still very raw; i.e. a VMSTATE_UINT32
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ends up with a 4 byte bigendian representation on the wire; in the future
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it might be possible to use a more structured format.
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Legacy way
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----------
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This way is going to disappear as soon as all current users are ported to VMSTATE;
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although converting existing code can be tricky, and thus 'soon' is relative.
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Each device has to register two functions, one to save the state and
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another to load the state back.
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.. code:: c
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int register_savevm_live(const char *idstr,
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int instance_id,
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int version_id,
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SaveVMHandlers *ops,
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void *opaque);
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Two functions in the ``ops`` structure are the ``save_state``
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and ``load_state`` functions. Notice that ``load_state`` receives a version_id
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parameter to know what state format is receiving. ``save_state`` doesn't
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have a version_id parameter because it always uses the latest version.
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Note that because the VMState macros still save the data in a raw
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format, in many cases it's possible to replace legacy code
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with a carefully constructed VMState description that matches the
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byte layout of the existing code.
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Changing migration data structures
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----------------------------------
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When we migrate a device, we save/load the state as a series
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of fields. Sometimes, due to bugs or new functionality, we need to
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change the state to store more/different information. Changing the migration
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state saved for a device can break migration compatibility unless
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care is taken to use the appropriate techniques. In general QEMU tries
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to maintain forward migration compatibility (i.e. migrating from
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QEMU n->n+1) and there are users who benefit from backward compatibility
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as well.
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Subsections
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-----------
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The most common structure change is adding new data, e.g. when adding
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a newer form of device, or adding that state that you previously
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forgot to migrate. This is best solved using a subsection.
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A subsection is "like" a device vmstate, but with a particularity, it
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has a Boolean function that tells if that values are needed to be sent
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or not. If this functions returns false, the subsection is not sent.
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Subsections have a unique name, that is looked for on the receiving
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side.
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On the receiving side, if we found a subsection for a device that we
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don't understand, we just fail the migration. If we understand all
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the subsections, then we load the state with success. There's no check
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that a subsection is loaded, so a newer QEMU that knows about a subsection
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can (with care) load a stream from an older QEMU that didn't send
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the subsection.
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If the new data is only needed in a rare case, then the subsection
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can be made conditional on that case and the migration will still
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succeed to older QEMUs in most cases. This is OK for data that's
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critical, but in some use cases it's preferred that the migration
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should succeed even with the data missing. To support this the
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subsection can be connected to a device property and from there
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to a versioned machine type.
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The 'pre_load' and 'post_load' functions on subsections are only
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called if the subsection is loaded.
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One important note is that the outer post_load() function is called "after"
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loading all subsections, because a newer subsection could change the same
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value that it uses. A flag, and the combination of outer pre_load and
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post_load can be used to detect whether a subsection was loaded, and to
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fall back on default behaviour when the subsection isn't present.
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Example:
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.. code:: c
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static bool ide_drive_pio_state_needed(void *opaque)
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{
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IDEState *s = opaque;
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return ((s->status & DRQ_STAT) != 0)
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|| (s->bus->error_status & BM_STATUS_PIO_RETRY);
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}
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const VMStateDescription vmstate_ide_drive_pio_state = {
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.name = "ide_drive/pio_state",
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.version_id = 1,
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.minimum_version_id = 1,
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.pre_save = ide_drive_pio_pre_save,
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.post_load = ide_drive_pio_post_load,
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.needed = ide_drive_pio_state_needed,
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.fields = (const VMStateField[]) {
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VMSTATE_INT32(req_nb_sectors, IDEState),
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VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
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vmstate_info_uint8, uint8_t),
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VMSTATE_INT32(cur_io_buffer_offset, IDEState),
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VMSTATE_INT32(cur_io_buffer_len, IDEState),
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VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
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VMSTATE_INT32(elementary_transfer_size, IDEState),
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VMSTATE_INT32(packet_transfer_size, IDEState),
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VMSTATE_END_OF_LIST()
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}
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};
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const VMStateDescription vmstate_ide_drive = {
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.name = "ide_drive",
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.version_id = 3,
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.minimum_version_id = 0,
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.post_load = ide_drive_post_load,
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.fields = (const VMStateField[]) {
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.... several fields ....
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VMSTATE_END_OF_LIST()
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},
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.subsections = (const VMStateDescription * const []) {
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&vmstate_ide_drive_pio_state,
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NULL
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}
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};
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Here we have a subsection for the pio state. We only need to
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save/send this state when we are in the middle of a pio operation
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(that is what ``ide_drive_pio_state_needed()`` checks). If DRQ_STAT is
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not enabled, the values on that fields are garbage and don't need to
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be sent.
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Connecting subsections to properties
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------------------------------------
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Using a condition function that checks a 'property' to determine whether
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to send a subsection allows backward migration compatibility when
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new subsections are added, especially when combined with versioned
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machine types.
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For example:
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a) Add a new property using ``DEFINE_PROP_BOOL`` - e.g. support-foo and
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default it to true.
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b) Add an entry to the ``hw_compat_`` for the previous version that sets
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the property to false.
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c) Add a static bool support_foo function that tests the property.
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d) Add a subsection with a .needed set to the support_foo function
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e) (potentially) Add an outer pre_load that sets up a default value
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for 'foo' to be used if the subsection isn't loaded.
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Now that subsection will not be generated when using an older
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machine type and the migration stream will be accepted by older
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QEMU versions.
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Not sending existing elements
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-----------------------------
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Sometimes members of the VMState are no longer needed:
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- removing them will break migration compatibility
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- making them version dependent and bumping the version will break backward migration
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compatibility.
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Adding a dummy field into the migration stream is normally the best way to preserve
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compatibility.
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If the field really does need to be removed then:
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a) Add a new property/compatibility/function in the same way for subsections above.
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b) replace the VMSTATE macro with the _TEST version of the macro, e.g.:
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``VMSTATE_UINT32(foo, barstruct)``
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becomes
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``VMSTATE_UINT32_TEST(foo, barstruct, pre_version_baz)``
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Sometime in the future when we no longer care about the ancient versions these can be killed off.
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Note that for backward compatibility it's important to fill in the structure with
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data that the destination will understand.
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Any difference in the predicates on the source and destination will end up
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with different fields being enabled and data being loaded into the wrong
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fields; for this reason conditional fields like this are very fragile.
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Versions
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--------
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Version numbers are intended for major incompatible changes to the
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migration of a device, and using them breaks backward-migration
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compatibility; in general most changes can be made by adding Subsections
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(see above) or _TEST macros (see above) which won't break compatibility.
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Each version is associated with a series of fields saved. The ``save_state`` always saves
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the state as the newer version. But ``load_state`` sometimes is able to
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load state from an older version.
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You can see that there are two version fields:
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- ``version_id``: the maximum version_id supported by VMState for that device.
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- ``minimum_version_id``: the minimum version_id that VMState is able to understand
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for that device.
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VMState is able to read versions from minimum_version_id to version_id.
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There are *_V* forms of many ``VMSTATE_`` macros to load fields for version dependent fields,
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e.g.
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.. code:: c
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VMSTATE_UINT16_V(ip_id, Slirp, 2),
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only loads that field for versions 2 and newer.
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Saving state will always create a section with the 'version_id' value
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and thus can't be loaded by any older QEMU.
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Massaging functions
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-------------------
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Sometimes, it is not enough to be able to save the state directly
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from one structure, we need to fill the correct values there. One
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example is when we are using kvm. Before saving the cpu state, we
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need to ask kvm to copy to QEMU the state that it is using. And the
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opposite when we are loading the state, we need a way to tell kvm to
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load the state for the cpu that we have just loaded from the QEMUFile.
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The functions to do that are inside a vmstate definition, and are called:
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- ``int (*pre_load)(void *opaque);``
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This function is called before we load the state of one device.
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- ``int (*post_load)(void *opaque, int version_id);``
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This function is called after we load the state of one device.
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- ``int (*pre_save)(void *opaque);``
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This function is called before we save the state of one device.
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- ``int (*post_save)(void *opaque);``
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This function is called after we save the state of one device
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(even upon failure, unless the call to pre_save returned an error).
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Example: You can look at hpet.c, that uses the first three functions
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to massage the state that is transferred.
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The ``VMSTATE_WITH_TMP`` macro may be useful when the migration
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data doesn't match the stored device data well; it allows an
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intermediate temporary structure to be populated with migration
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data and then transferred to the main structure.
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If you use memory API functions that update memory layout outside
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initialization (i.e., in response to a guest action), this is a strong
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indication that you need to call these functions in a ``post_load`` callback.
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Examples of such memory API functions are:
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- memory_region_add_subregion()
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- memory_region_del_subregion()
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- memory_region_set_readonly()
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- memory_region_set_nonvolatile()
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- memory_region_set_enabled()
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- memory_region_set_address()
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- memory_region_set_alias_offset()
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Iterative device migration
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--------------------------
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Some devices, such as RAM, Block storage or certain platform devices,
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have large amounts of data that would mean that the CPUs would be
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paused for too long if they were sent in one section. For these
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devices an *iterative* approach is taken.
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The iterative devices generally don't use VMState macros
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(although it may be possible in some cases) and instead use
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qemu_put_*/qemu_get_* macros to read/write data to the stream. Specialist
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versions exist for high bandwidth IO.
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An iterative device must provide:
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- A ``save_setup`` function that initialises the data structures and
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transmits a first section containing information on the device. In the
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case of RAM this transmits a list of RAMBlocks and sizes.
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- A ``load_setup`` function that initialises the data structures on the
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destination.
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- A ``state_pending_exact`` function that indicates how much more
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data we must save. The core migration code will use this to
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determine when to pause the CPUs and complete the migration.
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- A ``state_pending_estimate`` function that indicates how much more
|
|
data we must save. When the estimated amount is smaller than the
|
|
threshold, we call ``state_pending_exact``.
|
|
|
|
- A ``save_live_iterate`` function should send a chunk of data until
|
|
the point that stream bandwidth limits tell it to stop. Each call
|
|
generates one section.
|
|
|
|
- A ``save_live_complete_precopy`` function that must transmit the
|
|
last section for the device containing any remaining data.
|
|
|
|
- A ``load_state`` function used to load sections generated by
|
|
any of the save functions that generate sections.
|
|
|
|
- ``cleanup`` functions for both save and load that are called
|
|
at the end of migration.
|
|
|
|
Note that the contents of the sections for iterative migration tend
|
|
to be open-coded by the devices; care should be taken in parsing
|
|
the results and structuring the stream to make them easy to validate.
|
|
|
|
Device ordering
|
|
---------------
|
|
|
|
There are cases in which the ordering of device loading matters; for
|
|
example in some systems where a device may assert an interrupt during loading,
|
|
if the interrupt controller is loaded later then it might lose the state.
|
|
|
|
Some ordering is implicitly provided by the order in which the machine
|
|
definition creates devices, however this is somewhat fragile.
|
|
|
|
The ``MigrationPriority`` enum provides a means of explicitly enforcing
|
|
ordering. Numerically higher priorities are loaded earlier.
|
|
The priority is set by setting the ``priority`` field of the top level
|
|
``VMStateDescription`` for the device.
|
|
|
|
Stream structure
|
|
================
|
|
|
|
The stream tries to be word and endian agnostic, allowing migration between hosts
|
|
of different characteristics running the same VM.
|
|
|
|
- Header
|
|
|
|
- Magic
|
|
- Version
|
|
- VM configuration section
|
|
|
|
- Machine type
|
|
- Target page bits
|
|
- List of sections
|
|
Each section contains a device, or one iteration of a device save.
|
|
|
|
- section type
|
|
- section id
|
|
- ID string (First section of each device)
|
|
- instance id (First section of each device)
|
|
- version id (First section of each device)
|
|
- <device data>
|
|
- Footer mark
|
|
- EOF mark
|
|
- VM Description structure
|
|
Consisting of a JSON description of the contents for analysis only
|
|
|
|
The ``device data`` in each section consists of the data produced
|
|
by the code described above. For non-iterative devices they have a single
|
|
section; iterative devices have an initial and last section and a set
|
|
of parts in between.
|
|
Note that there is very little checking by the common code of the integrity
|
|
of the ``device data`` contents, that's up to the devices themselves.
|
|
The ``footer mark`` provides a little bit of protection for the case where
|
|
the receiving side reads more or less data than expected.
|
|
|
|
The ``ID string`` is normally unique, having been formed from a bus name
|
|
and device address, PCI devices and storage devices hung off PCI controllers
|
|
fit this pattern well. Some devices are fixed single instances (e.g. "pc-ram").
|
|
Others (especially either older devices or system devices which for
|
|
some reason don't have a bus concept) make use of the ``instance id``
|
|
for otherwise identically named devices.
|
|
|
|
Return path
|
|
-----------
|
|
|
|
Only a unidirectional stream is required for normal migration, however a
|
|
``return path`` can be created when bidirectional communication is desired.
|
|
This is primarily used by postcopy, but is also used to return a success
|
|
flag to the source at the end of migration.
|
|
|
|
``qemu_file_get_return_path(QEMUFile* fwdpath)`` gives the QEMUFile* for the return
|
|
path.
|
|
|
|
Source side
|
|
|
|
Forward path - written by migration thread
|
|
Return path - opened by main thread, read by return-path thread
|
|
|
|
Destination side
|
|
|
|
Forward path - read by main thread
|
|
Return path - opened by main thread, written by main thread AND postcopy
|
|
thread (protected by rp_mutex)
|
|
|
|
Dirty limit
|
|
=====================
|
|
The dirty limit, short for dirty page rate upper limit, is a new capability
|
|
introduced in the 8.1 QEMU release that uses a new algorithm based on the KVM
|
|
dirty ring to throttle down the guest during live migration.
|
|
|
|
The algorithm framework is as follows:
|
|
|
|
::
|
|
|
|
------------------------------------------------------------------------------
|
|
main --------------> throttle thread ------------> PREPARE(1) <--------
|
|
thread \ | |
|
|
\ | |
|
|
\ V |
|
|
-\ CALCULATE(2) |
|
|
\ | |
|
|
\ | |
|
|
\ V |
|
|
\ SET PENALTY(3) -----
|
|
-\ |
|
|
\ |
|
|
\ V
|
|
-> virtual CPU thread -------> ACCEPT PENALTY(4)
|
|
------------------------------------------------------------------------------
|
|
|
|
When the qmp command qmp_set_vcpu_dirty_limit is called for the first time,
|
|
the QEMU main thread starts the throttle thread. The throttle thread, once
|
|
launched, executes the loop, which consists of three steps:
|
|
|
|
- PREPARE (1)
|
|
|
|
The entire work of PREPARE (1) is preparation for the second stage,
|
|
CALCULATE(2), as the name implies. It involves preparing the dirty
|
|
page rate value and the corresponding upper limit of the VM:
|
|
The dirty page rate is calculated via the KVM dirty ring mechanism,
|
|
which tells QEMU how many dirty pages a virtual CPU has had since the
|
|
last KVM_EXIT_DIRTY_RING_FULL exception; The dirty page rate upper
|
|
limit is specified by caller, therefore fetch it directly.
|
|
|
|
- CALCULATE (2)
|
|
|
|
Calculate a suitable sleep period for each virtual CPU, which will be
|
|
used to determine the penalty for the target virtual CPU. The
|
|
computation must be done carefully in order to reduce the dirty page
|
|
rate progressively down to the upper limit without oscillation. To
|
|
achieve this, two strategies are provided: the first is to add or
|
|
subtract sleep time based on the ratio of the current dirty page rate
|
|
to the limit, which is used when the current dirty page rate is far
|
|
from the limit; the second is to add or subtract a fixed time when
|
|
the current dirty page rate is close to the limit.
|
|
|
|
- SET PENALTY (3)
|
|
|
|
Set the sleep time for each virtual CPU that should be penalized based
|
|
on the results of the calculation supplied by step CALCULATE (2).
|
|
|
|
After completing the three above stages, the throttle thread loops back
|
|
to step PREPARE (1) until the dirty limit is reached.
|
|
|
|
On the other hand, each virtual CPU thread reads the sleep duration and
|
|
sleeps in the path of the KVM_EXIT_DIRTY_RING_FULL exception handler, that
|
|
is ACCEPT PENALTY (4). Virtual CPUs tied with writing processes will
|
|
obviously exit to the path and get penalized, whereas virtual CPUs involved
|
|
with read processes will not.
|
|
|
|
In summary, thanks to the KVM dirty ring technology, the dirty limit
|
|
algorithm will restrict virtual CPUs as needed to keep their dirty page
|
|
rate inside the limit. This leads to more steady reading performance during
|
|
live migration and can aid in improving large guest responsiveness.
|
|
|
|
Postcopy
|
|
========
|
|
|
|
'Postcopy' migration is a way to deal with migrations that refuse to converge
|
|
(or take too long to converge) its plus side is that there is an upper bound on
|
|
the amount of migration traffic and time it takes, the down side is that during
|
|
the postcopy phase, a failure of *either* side causes the guest to be lost.
|
|
|
|
In postcopy the destination CPUs are started before all the memory has been
|
|
transferred, and accesses to pages that are yet to be transferred cause
|
|
a fault that's translated by QEMU into a request to the source QEMU.
|
|
|
|
Postcopy can be combined with precopy (i.e. normal migration) so that if precopy
|
|
doesn't finish in a given time the switch is made to postcopy.
|
|
|
|
Enabling postcopy
|
|
-----------------
|
|
|
|
To enable postcopy, issue this command on the monitor (both source and
|
|
destination) prior to the start of migration:
|
|
|
|
``migrate_set_capability postcopy-ram on``
|
|
|
|
The normal commands are then used to start a migration, which is still
|
|
started in precopy mode. Issuing:
|
|
|
|
``migrate_start_postcopy``
|
|
|
|
will now cause the transition from precopy to postcopy.
|
|
It can be issued immediately after migration is started or any
|
|
time later on. Issuing it after the end of a migration is harmless.
|
|
|
|
Blocktime is a postcopy live migration metric, intended to show how
|
|
long the vCPU was in state of interruptible sleep due to pagefault.
|
|
That metric is calculated both for all vCPUs as overlapped value, and
|
|
separately for each vCPU. These values are calculated on destination
|
|
side. To enable postcopy blocktime calculation, enter following
|
|
command on destination monitor:
|
|
|
|
``migrate_set_capability postcopy-blocktime on``
|
|
|
|
Postcopy blocktime can be retrieved by query-migrate qmp command.
|
|
postcopy-blocktime value of qmp command will show overlapped blocking
|
|
time for all vCPU, postcopy-vcpu-blocktime will show list of blocking
|
|
time per vCPU.
|
|
|
|
.. note::
|
|
During the postcopy phase, the bandwidth limits set using
|
|
``migrate_set_parameter`` is ignored (to avoid delaying requested pages that
|
|
the destination is waiting for).
|
|
|
|
Postcopy device transfer
|
|
------------------------
|
|
|
|
Loading of device data may cause the device emulation to access guest RAM
|
|
that may trigger faults that have to be resolved by the source, as such
|
|
the migration stream has to be able to respond with page data *during* the
|
|
device load, and hence the device data has to be read from the stream completely
|
|
before the device load begins to free the stream up. This is achieved by
|
|
'packaging' the device data into a blob that's read in one go.
|
|
|
|
Source behaviour
|
|
----------------
|
|
|
|
Until postcopy is entered the migration stream is identical to normal
|
|
precopy, except for the addition of a 'postcopy advise' command at
|
|
the beginning, to tell the destination that postcopy might happen.
|
|
When postcopy starts the source sends the page discard data and then
|
|
forms the 'package' containing:
|
|
|
|
- Command: 'postcopy listen'
|
|
- The device state
|
|
|
|
A series of sections, identical to the precopy streams device state stream
|
|
containing everything except postcopiable devices (i.e. RAM)
|
|
- Command: 'postcopy run'
|
|
|
|
The 'package' is sent as the data part of a Command: ``CMD_PACKAGED``, and the
|
|
contents are formatted in the same way as the main migration stream.
|
|
|
|
During postcopy the source scans the list of dirty pages and sends them
|
|
to the destination without being requested (in much the same way as precopy),
|
|
however when a page request is received from the destination, the dirty page
|
|
scanning restarts from the requested location. This causes requested pages
|
|
to be sent quickly, and also causes pages directly after the requested page
|
|
to be sent quickly in the hope that those pages are likely to be used
|
|
by the destination soon.
|
|
|
|
Destination behaviour
|
|
---------------------
|
|
|
|
Initially the destination looks the same as precopy, with a single thread
|
|
reading the migration stream; the 'postcopy advise' and 'discard' commands
|
|
are processed to change the way RAM is managed, but don't affect the stream
|
|
processing.
|
|
|
|
::
|
|
|
|
------------------------------------------------------------------------------
|
|
1 2 3 4 5 6 7
|
|
main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
|
|
thread | |
|
|
| (page request)
|
|
| \___
|
|
v \
|
|
listen thread: --- page -- page -- page -- page -- page --
|
|
|
|
a b c
|
|
------------------------------------------------------------------------------
|
|
|
|
- On receipt of ``CMD_PACKAGED`` (1)
|
|
|
|
All the data associated with the package - the ( ... ) section in the diagram -
|
|
is read into memory, and the main thread recurses into qemu_loadvm_state_main
|
|
to process the contents of the package (2) which contains commands (3,6) and
|
|
devices (4...)
|
|
|
|
- On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
|
|
|
|
a new thread (a) is started that takes over servicing the migration stream,
|
|
while the main thread carries on loading the package. It loads normal
|
|
background page data (b) but if during a device load a fault happens (5)
|
|
the returned page (c) is loaded by the listen thread allowing the main
|
|
threads device load to carry on.
|
|
|
|
- The last thing in the ``CMD_PACKAGED`` is a 'RUN' command (6)
|
|
|
|
letting the destination CPUs start running. At the end of the
|
|
``CMD_PACKAGED`` (7) the main thread returns to normal running behaviour and
|
|
is no longer used by migration, while the listen thread carries on servicing
|
|
page data until the end of migration.
|
|
|
|
Postcopy Recovery
|
|
-----------------
|
|
|
|
Comparing to precopy, postcopy is special on error handlings. When any
|
|
error happens (in this case, mostly network errors), QEMU cannot easily
|
|
fail a migration because VM data resides in both source and destination
|
|
QEMU instances. On the other hand, when issue happens QEMU on both sides
|
|
will go into a paused state. It'll need a recovery phase to continue a
|
|
paused postcopy migration.
|
|
|
|
The recovery phase normally contains a few steps:
|
|
|
|
- When network issue occurs, both QEMU will go into PAUSED state
|
|
|
|
- When the network is recovered (or a new network is provided), the admin
|
|
can setup the new channel for migration using QMP command
|
|
'migrate-recover' on destination node, preparing for a resume.
|
|
|
|
- On source host, the admin can continue the interrupted postcopy
|
|
migration using QMP command 'migrate' with resume=true flag set.
|
|
|
|
- After the connection is re-established, QEMU will continue the postcopy
|
|
migration on both sides.
|
|
|
|
During a paused postcopy migration, the VM can logically still continue
|
|
running, and it will not be impacted from any page access to pages that
|
|
were already migrated to destination VM before the interruption happens.
|
|
However, if any of the missing pages got accessed on destination VM, the VM
|
|
thread will be halted waiting for the page to be migrated, it means it can
|
|
be halted until the recovery is complete.
|
|
|
|
The impact of accessing missing pages can be relevant to different
|
|
configurations of the guest. For example, when with async page fault
|
|
enabled, logically the guest can proactively schedule out the threads
|
|
accessing missing pages.
|
|
|
|
Postcopy states
|
|
---------------
|
|
|
|
Postcopy moves through a series of states (see postcopy_state) from
|
|
ADVISE->DISCARD->LISTEN->RUNNING->END
|
|
|
|
- Advise
|
|
|
|
Set at the start of migration if postcopy is enabled, even
|
|
if it hasn't had the start command; here the destination
|
|
checks that its OS has the support needed for postcopy, and performs
|
|
setup to ensure the RAM mappings are suitable for later postcopy.
|
|
The destination will fail early in migration at this point if the
|
|
required OS support is not present.
|
|
(Triggered by reception of POSTCOPY_ADVISE command)
|
|
|
|
- Discard
|
|
|
|
Entered on receipt of the first 'discard' command; prior to
|
|
the first Discard being performed, hugepages are switched off
|
|
(using madvise) to ensure that no new huge pages are created
|
|
during the postcopy phase, and to cause any huge pages that
|
|
have discards on them to be broken.
|
|
|
|
- Listen
|
|
|
|
The first command in the package, POSTCOPY_LISTEN, switches
|
|
the destination state to Listen, and starts a new thread
|
|
(the 'listen thread') which takes over the job of receiving
|
|
pages off the migration stream, while the main thread carries
|
|
on processing the blob. With this thread able to process page
|
|
reception, the destination now 'sensitises' the RAM to detect
|
|
any access to missing pages (on Linux using the 'userfault'
|
|
system).
|
|
|
|
- Running
|
|
|
|
POSTCOPY_RUN causes the destination to synchronise all
|
|
state and start the CPUs and IO devices running. The main
|
|
thread now finishes processing the migration package and
|
|
now carries on as it would for normal precopy migration
|
|
(although it can't do the cleanup it would do as it
|
|
finishes a normal migration).
|
|
|
|
- Paused
|
|
|
|
Postcopy can run into a paused state (normally on both sides when
|
|
happens), where all threads will be temporarily halted mostly due to
|
|
network errors. When reaching paused state, migration will make sure
|
|
the qemu binary on both sides maintain the data without corrupting
|
|
the VM. To continue the migration, the admin needs to fix the
|
|
migration channel using the QMP command 'migrate-recover' on the
|
|
destination node, then resume the migration using QMP command 'migrate'
|
|
again on source node, with resume=true flag set.
|
|
|
|
- End
|
|
|
|
The listen thread can now quit, and perform the cleanup of migration
|
|
state, the migration is now complete.
|
|
|
|
Source side page map
|
|
--------------------
|
|
|
|
The 'migration bitmap' in postcopy is basically the same as in the precopy,
|
|
where each of the bit to indicate that page is 'dirty' - i.e. needs
|
|
sending. During the precopy phase this is updated as the CPU dirties
|
|
pages, however during postcopy the CPUs are stopped and nothing should
|
|
dirty anything any more. Instead, dirty bits are cleared when the relevant
|
|
pages are sent during postcopy.
|
|
|
|
Postcopy with hugepages
|
|
-----------------------
|
|
|
|
Postcopy now works with hugetlbfs backed memory:
|
|
|
|
a) The linux kernel on the destination must support userfault on hugepages.
|
|
b) The huge-page configuration on the source and destination VMs must be
|
|
identical; i.e. RAMBlocks on both sides must use the same page size.
|
|
c) Note that ``-mem-path /dev/hugepages`` will fall back to allocating normal
|
|
RAM if it doesn't have enough hugepages, triggering (b) to fail.
|
|
Using ``-mem-prealloc`` enforces the allocation using hugepages.
|
|
d) Care should be taken with the size of hugepage used; postcopy with 2MB
|
|
hugepages works well, however 1GB hugepages are likely to be problematic
|
|
since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,
|
|
and until the full page is transferred the destination thread is blocked.
|
|
|
|
Postcopy with shared memory
|
|
---------------------------
|
|
|
|
Postcopy migration with shared memory needs explicit support from the other
|
|
processes that share memory and from QEMU. There are restrictions on the type of
|
|
memory that userfault can support shared.
|
|
|
|
The Linux kernel userfault support works on ``/dev/shm`` memory and on ``hugetlbfs``
|
|
(although the kernel doesn't provide an equivalent to ``madvise(MADV_DONTNEED)``
|
|
for hugetlbfs which may be a problem in some configurations).
|
|
|
|
The vhost-user code in QEMU supports clients that have Postcopy support,
|
|
and the ``vhost-user-bridge`` (in ``tests/``) and the DPDK package have changes
|
|
to support postcopy.
|
|
|
|
The client needs to open a userfaultfd and register the areas
|
|
of memory that it maps with userfault. The client must then pass the
|
|
userfaultfd back to QEMU together with a mapping table that allows
|
|
fault addresses in the clients address space to be converted back to
|
|
RAMBlock/offsets. The client's userfaultfd is added to the postcopy
|
|
fault-thread and page requests are made on behalf of the client by QEMU.
|
|
QEMU performs 'wake' operations on the client's userfaultfd to allow it
|
|
to continue after a page has arrived.
|
|
|
|
.. note::
|
|
There are two future improvements that would be nice:
|
|
a) Some way to make QEMU ignorant of the addresses in the clients
|
|
address space
|
|
b) Avoiding the need for QEMU to perform ufd-wake calls after the
|
|
pages have arrived
|
|
|
|
Retro-fitting postcopy to existing clients is possible:
|
|
a) A mechanism is needed for the registration with userfault as above,
|
|
and the registration needs to be coordinated with the phases of
|
|
postcopy. In vhost-user extra messages are added to the existing
|
|
control channel.
|
|
b) Any thread that can block due to guest memory accesses must be
|
|
identified and the implication understood; for example if the
|
|
guest memory access is made while holding a lock then all other
|
|
threads waiting for that lock will also be blocked.
|
|
|
|
Postcopy Preemption Mode
|
|
------------------------
|
|
|
|
Postcopy preempt is a new capability introduced in 8.0 QEMU release, it
|
|
allows urgent pages (those got page fault requested from destination QEMU
|
|
explicitly) to be sent in a separate preempt channel, rather than queued in
|
|
the background migration channel. Anyone who cares about latencies of page
|
|
faults during a postcopy migration should enable this feature. By default,
|
|
it's not enabled.
|
|
|
|
Firmware
|
|
========
|
|
|
|
Migration migrates the copies of RAM and ROM, and thus when running
|
|
on the destination it includes the firmware from the source. Even after
|
|
resetting a VM, the old firmware is used. Only once QEMU has been restarted
|
|
is the new firmware in use.
|
|
|
|
- Changes in firmware size can cause changes in the required RAMBlock size
|
|
to hold the firmware and thus migration can fail. In practice it's best
|
|
to pad firmware images to convenient powers of 2 with plenty of space
|
|
for growth.
|
|
|
|
- Care should be taken with device emulation code so that newer
|
|
emulation code can work with older firmware to allow forward migration.
|
|
|
|
- Care should be taken with newer firmware so that backward migration
|
|
to older systems with older device emulation code will work.
|
|
|
|
In some cases it may be best to tie specific firmware versions to specific
|
|
versioned machine types to cut down on the combinations that will need
|
|
support. This is also useful when newer versions of firmware outgrow
|
|
the padding.
|
|
|
|
|
|
Backwards compatibility
|
|
=======================
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How backwards compatibility works
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---------------------------------
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When we do migration, we have two QEMU processes: the source and the
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target. There are two cases, they are the same version or they are
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different versions. The easy case is when they are the same version.
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The difficult one is when they are different versions.
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There are two things that are different, but they have very similar
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names and sometimes get confused:
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- QEMU version
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- machine type version
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Let's start with a practical example, we start with:
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- qemu-system-x86_64 (v5.2), from now on qemu-5.2.
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- qemu-system-x86_64 (v5.1), from now on qemu-5.1.
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Related to this are the "latest" machine types defined on each of
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them:
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- pc-q35-5.2 (newer one in qemu-5.2) from now on pc-5.2
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- pc-q35-5.1 (newer one in qemu-5.1) from now on pc-5.1
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First of all, migration is only supposed to work if you use the same
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machine type in both source and destination. The QEMU hardware
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configuration needs to be the same also on source and destination.
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Most aspects of the backend configuration can be changed at will,
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except for a few cases where the backend features influence frontend
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device feature exposure. But that is not relevant for this section.
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I am going to list the number of combinations that we can have. Let's
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start with the trivial ones, QEMU is the same on source and
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destination:
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1 - qemu-5.2 -M pc-5.2 -> migrates to -> qemu-5.2 -M pc-5.2
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This is the latest QEMU with the latest machine type.
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This have to work, and if it doesn't work it is a bug.
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2 - qemu-5.1 -M pc-5.1 -> migrates to -> qemu-5.1 -M pc-5.1
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Exactly the same case than the previous one, but for 5.1.
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Nothing to see here either.
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This are the easiest ones, we will not talk more about them in this
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section.
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Now we start with the more interesting cases. Consider the case where
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we have the same QEMU version in both sides (qemu-5.2) but we are using
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the latest machine type for that version (pc-5.2) but one of an older
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QEMU version, in this case pc-5.1.
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3 - qemu-5.2 -M pc-5.1 -> migrates to -> qemu-5.2 -M pc-5.1
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It needs to use the definition of pc-5.1 and the devices as they
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were configured on 5.1, but this should be easy in the sense that
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both sides are the same QEMU and both sides have exactly the same
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idea of what the pc-5.1 machine is.
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4 - qemu-5.1 -M pc-5.2 -> migrates to -> qemu-5.1 -M pc-5.2
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This combination is not possible as the qemu-5.1 doesn't understand
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pc-5.2 machine type. So nothing to worry here.
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Now it comes the interesting ones, when both QEMU processes are
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different. Notice also that the machine type needs to be pc-5.1,
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because we have the limitation than qemu-5.1 doesn't know pc-5.2. So
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the possible cases are:
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5 - qemu-5.2 -M pc-5.1 -> migrates to -> qemu-5.1 -M pc-5.1
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This migration is known as newer to older. We need to make sure
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when we are developing 5.2 we need to take care about not to break
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migration to qemu-5.1. Notice that we can't make updates to
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qemu-5.1 to understand whatever qemu-5.2 decides to change, so it is
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in qemu-5.2 side to make the relevant changes.
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6 - qemu-5.1 -M pc-5.1 -> migrates to -> qemu-5.2 -M pc-5.1
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This migration is known as older to newer. We need to make sure
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than we are able to receive migrations from qemu-5.1. The problem is
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similar to the previous one.
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If qemu-5.1 and qemu-5.2 were the same, there will not be any
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compatibility problems. But the reason that we create qemu-5.2 is to
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get new features, devices, defaults, etc.
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If we get a device that has a new feature, or change a default value,
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we have a problem when we try to migrate between different QEMU
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versions.
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So we need a way to tell qemu-5.2 that when we are using machine type
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pc-5.1, it needs to **not** use the feature, to be able to migrate to
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real qemu-5.1.
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And the equivalent part when migrating from qemu-5.1 to qemu-5.2.
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qemu-5.2 has to expect that it is not going to get data for the new
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feature, because qemu-5.1 doesn't know about it.
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How do we tell QEMU about these device feature changes? In
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hw/core/machine.c:hw_compat_X_Y arrays.
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If we change a default value, we need to put back the old value on
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that array. And the device, during initialization needs to look at
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that array to see what value it needs to get for that feature. And
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what are we going to put in that array, the value of a property.
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To create a property for a device, we need to use one of the
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DEFINE_PROP_*() macros. See include/hw/qdev-properties.h to find the
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macros that exist. With it, we set the default value for that
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property, and that is what it is going to get in the latest released
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version. But if we want a different value for a previous version, we
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can change that in the hw_compat_X_Y arrays.
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hw_compat_X_Y is an array of registers that have the format:
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- name_device
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- name_property
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- value
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Let's see a practical example.
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In qemu-5.2 virtio-blk-device got multi queue support. This is a
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change that is not backward compatible. In qemu-5.1 it has one
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queue. In qemu-5.2 it has the same number of queues as the number of
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cpus in the system.
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When we are doing migration, if we migrate from a device that has 4
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queues to a device that have only one queue, we don't know where to
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put the extra information for the other 3 queues, and we fail
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migration.
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Similar problem when we migrate from qemu-5.1 that has only one queue
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to qemu-5.2, we only sent information for one queue, but destination
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has 4, and we have 3 queues that are not properly initialized and
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anything can happen.
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So, how can we address this problem. Easy, just convince qemu-5.2
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that when it is running pc-5.1, it needs to set the number of queues
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for virtio-blk-devices to 1.
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That way we fix the cases 5 and 6.
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5 - qemu-5.2 -M pc-5.1 -> migrates to -> qemu-5.1 -M pc-5.1
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qemu-5.2 -M pc-5.1 sets number of queues to be 1.
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qemu-5.1 -M pc-5.1 expects number of queues to be 1.
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correct. migration works.
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6 - qemu-5.1 -M pc-5.1 -> migrates to -> qemu-5.2 -M pc-5.1
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qemu-5.1 -M pc-5.1 sets number of queues to be 1.
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qemu-5.2 -M pc-5.1 expects number of queues to be 1.
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correct. migration works.
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And now the other interesting case, case 3. In this case we have:
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3 - qemu-5.2 -M pc-5.1 -> migrates to -> qemu-5.2 -M pc-5.1
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Here we have the same QEMU in both sides. So it doesn't matter a
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lot if we have set the number of queues to 1 or not, because
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they are the same.
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WRONG!
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Think what happens if we do one of this double migrations:
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A -> migrates -> B -> migrates -> C
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where:
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A: qemu-5.1 -M pc-5.1
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B: qemu-5.2 -M pc-5.1
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C: qemu-5.2 -M pc-5.1
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migration A -> B is case 6, so number of queues needs to be 1.
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migration B -> C is case 3, so we don't care. But actually we
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care because we haven't started the guest in qemu-5.2, it came
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migrated from qemu-5.1. So to be in the safe place, we need to
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always use number of queues 1 when we are using pc-5.1.
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Now, how was this done in reality? The following commit shows how it
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was done::
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commit 9445e1e15e66c19e42bea942ba810db28052cd05
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Author: Stefan Hajnoczi <stefanha@redhat.com>
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Date: Tue Aug 18 15:33:47 2020 +0100
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virtio-blk-pci: default num_queues to -smp N
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The relevant parts for migration are::
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@@ -1281,7 +1284,8 @@ static Property virtio_blk_properties[] = {
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#endif
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DEFINE_PROP_BIT("request-merging", VirtIOBlock, conf.request_merging, 0,
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true),
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- DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues, 1),
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+ DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues,
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+ VIRTIO_BLK_AUTO_NUM_QUEUES),
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DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 256),
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It changes the default value of num_queues. But it fishes it for old
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machine types to have the right value::
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@@ -31,6 +31,7 @@
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GlobalProperty hw_compat_5_1[] = {
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...
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+ { "virtio-blk-device", "num-queues", "1"},
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...
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};
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A device with different features on both sides
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----------------------------------------------
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Let's assume that we are using the same QEMU binary on both sides,
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just to make the things easier. But we have a device that has
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different features on both sides of the migration. That can be
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because the devices are different, because the kernel driver of both
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devices have different features, whatever.
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How can we get this to work with migration. The way to do that is
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"theoretically" easy. You have to get the features that the device
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has in the source of the migration. The features that the device has
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on the target of the migration, you get the intersection of the
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features of both sides, and that is the way that you should launch
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QEMU.
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Notice that this is not completely related to QEMU. The most
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important thing here is that this should be handled by the managing
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application that launches QEMU. If QEMU is configured correctly, the
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migration will succeed.
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That said, actually doing it is complicated. Almost all devices are
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bad at being able to be launched with only some features enabled.
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With one big exception: cpus.
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You can read the documentation for QEMU x86 cpu models here:
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https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html
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See when they talk about migration they recommend that one chooses the
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newest cpu model that is supported for all cpus.
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Let's say that we have:
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Host A:
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Device X has the feature Y
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Host B:
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Device X has not the feature Y
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If we try to migrate without any care from host A to host B, it will
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fail because when migration tries to load the feature Y on
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destination, it will find that the hardware is not there.
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Doing this would be the equivalent of doing with cpus:
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Host A:
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$ qemu-system-x86_64 -cpu host
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Host B:
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$ qemu-system-x86_64 -cpu host
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When both hosts have different cpu features this is guaranteed to
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fail. Especially if Host B has less features than host A. If host A
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has less features than host B, sometimes it works. Important word of
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last sentence is "sometimes".
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So, forgetting about cpu models and continuing with the -cpu host
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example, let's see that the differences of the cpus is that Host A and
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B have the following features:
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Features: 'pcid' 'stibp' 'taa-no'
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Host A: X X
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Host B: X
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And we want to migrate between them, the way configure both QEMU cpu
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will be:
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Host A:
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$ qemu-system-x86_64 -cpu host,pcid=off,stibp=off
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Host B:
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$ qemu-system-x86_64 -cpu host,taa-no=off
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And you would be able to migrate between them. It is responsibility
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of the management application or of the user to make sure that the
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configuration is correct. QEMU doesn't know how to look at this kind
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of features in general.
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Notice that we don't recommend to use -cpu host for migration. It is
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used in this example because it makes the example simpler.
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Other devices have worse control about individual features. If they
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want to be able to migrate between hosts that show different features,
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the device needs a way to configure which ones it is going to use.
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In this section we have considered that we are using the same QEMU
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binary in both sides of the migration. If we use different QEMU
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versions process, then we need to have into account all other
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differences and the examples become even more complicated.
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How to mitigate when we have a backward compatibility error
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-----------------------------------------------------------
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We broke migration for old machine types continuously during
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development. But as soon as we find that there is a problem, we fix
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it. The problem is what happens when we detect after we have done a
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release that something has gone wrong.
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Let see how it worked with one example.
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After the release of qemu-8.0 we found a problem when doing migration
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of the machine type pc-7.2.
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- $ qemu-7.2 -M pc-7.2 -> qemu-7.2 -M pc-7.2
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This migration works
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- $ qemu-8.0 -M pc-7.2 -> qemu-8.0 -M pc-7.2
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This migration works
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- $ qemu-8.0 -M pc-7.2 -> qemu-7.2 -M pc-7.2
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This migration fails
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- $ qemu-7.2 -M pc-7.2 -> qemu-8.0 -M pc-7.2
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This migration fails
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So clearly something fails when migration between qemu-7.2 and
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qemu-8.0 with machine type pc-7.2. The error messages, and git bisect
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pointed to this commit.
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In qemu-8.0 we got this commit::
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commit 010746ae1db7f52700cb2e2c46eb94f299cfa0d2
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Author: Jonathan Cameron <Jonathan.Cameron@huawei.com>
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Date: Thu Mar 2 13:37:02 2023 +0000
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hw/pci/aer: Implement PCI_ERR_UNCOR_MASK register
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The relevant bits of the commit for our example are this ones::
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--- a/hw/pci/pcie_aer.c
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+++ b/hw/pci/pcie_aer.c
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@@ -112,6 +112,10 @@ int pcie_aer_init(PCIDevice *dev,
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pci_set_long(dev->w1cmask + offset + PCI_ERR_UNCOR_STATUS,
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PCI_ERR_UNC_SUPPORTED);
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+ pci_set_long(dev->config + offset + PCI_ERR_UNCOR_MASK,
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+ PCI_ERR_UNC_MASK_DEFAULT);
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+ pci_set_long(dev->wmask + offset + PCI_ERR_UNCOR_MASK,
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+ PCI_ERR_UNC_SUPPORTED);
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pci_set_long(dev->config + offset + PCI_ERR_UNCOR_SEVER,
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PCI_ERR_UNC_SEVERITY_DEFAULT);
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The patch changes how we configure PCI space for AER. But QEMU fails
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when the PCI space configuration is different between source and
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destination.
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The following commit shows how this got fixed::
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commit 5ed3dabe57dd9f4c007404345e5f5bf0e347317f
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Author: Leonardo Bras <leobras@redhat.com>
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Date: Tue May 2 21:27:02 2023 -0300
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hw/pci: Disable PCI_ERR_UNCOR_MASK register for machine type < 8.0
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[...]
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The relevant parts of the fix in QEMU are as follow:
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First, we create a new property for the device to be able to configure
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the old behaviour or the new behaviour::
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diff --git a/hw/pci/pci.c b/hw/pci/pci.c
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index 8a87ccc8b0..5153ad63d6 100644
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--- a/hw/pci/pci.c
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+++ b/hw/pci/pci.c
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@@ -79,6 +79,8 @@ static Property pci_props[] = {
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DEFINE_PROP_STRING("failover_pair_id", PCIDevice,
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failover_pair_id),
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DEFINE_PROP_UINT32("acpi-index", PCIDevice, acpi_index, 0),
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+ DEFINE_PROP_BIT("x-pcie-err-unc-mask", PCIDevice, cap_present,
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+ QEMU_PCIE_ERR_UNC_MASK_BITNR, true),
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DEFINE_PROP_END_OF_LIST()
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};
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Notice that we enable the feature for new machine types.
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Now we see how the fix is done. This is going to depend on what kind
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of breakage happens, but in this case it is quite simple::
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diff --git a/hw/pci/pcie_aer.c b/hw/pci/pcie_aer.c
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index 103667c368..374d593ead 100644
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--- a/hw/pci/pcie_aer.c
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+++ b/hw/pci/pcie_aer.c
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@@ -112,10 +112,13 @@ int pcie_aer_init(PCIDevice *dev, uint8_t cap_ver,
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uint16_t offset,
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pci_set_long(dev->w1cmask + offset + PCI_ERR_UNCOR_STATUS,
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PCI_ERR_UNC_SUPPORTED);
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- pci_set_long(dev->config + offset + PCI_ERR_UNCOR_MASK,
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- PCI_ERR_UNC_MASK_DEFAULT);
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- pci_set_long(dev->wmask + offset + PCI_ERR_UNCOR_MASK,
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- PCI_ERR_UNC_SUPPORTED);
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+
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+ if (dev->cap_present & QEMU_PCIE_ERR_UNC_MASK) {
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+ pci_set_long(dev->config + offset + PCI_ERR_UNCOR_MASK,
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+ PCI_ERR_UNC_MASK_DEFAULT);
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+ pci_set_long(dev->wmask + offset + PCI_ERR_UNCOR_MASK,
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+ PCI_ERR_UNC_SUPPORTED);
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+ }
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pci_set_long(dev->config + offset + PCI_ERR_UNCOR_SEVER,
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PCI_ERR_UNC_SEVERITY_DEFAULT);
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I.e. If the property bit is enabled, we configure it as we did for
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qemu-8.0. If the property bit is not set, we configure it as it was in 7.2.
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And now, everything that is missing is disabling the feature for old
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machine types::
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diff --git a/hw/core/machine.c b/hw/core/machine.c
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index 47a34841a5..07f763eb2e 100644
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--- a/hw/core/machine.c
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+++ b/hw/core/machine.c
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@@ -48,6 +48,7 @@ GlobalProperty hw_compat_7_2[] = {
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{ "e1000e", "migrate-timadj", "off" },
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{ "virtio-mem", "x-early-migration", "false" },
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{ "migration", "x-preempt-pre-7-2", "true" },
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+ { TYPE_PCI_DEVICE, "x-pcie-err-unc-mask", "off" },
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};
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const size_t hw_compat_7_2_len = G_N_ELEMENTS(hw_compat_7_2);
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And now, when qemu-8.0.1 is released with this fix, all combinations
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are going to work as supposed.
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- $ qemu-7.2 -M pc-7.2 -> qemu-7.2 -M pc-7.2 (works)
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- $ qemu-8.0.1 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 (works)
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- $ qemu-8.0.1 -M pc-7.2 -> qemu-7.2 -M pc-7.2 (works)
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- $ qemu-7.2 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 (works)
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So the normality has been restored and everything is ok, no?
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Not really, now our matrix is much bigger. We started with the easy
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cases, migration from the same version to the same version always
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works:
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- $ qemu-7.2 -M pc-7.2 -> qemu-7.2 -M pc-7.2
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- $ qemu-8.0 -M pc-7.2 -> qemu-8.0 -M pc-7.2
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- $ qemu-8.0.1 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2
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Now the interesting ones. When the QEMU processes versions are
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different. For the 1st set, their fail and we can do nothing, both
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versions are released and we can't change anything.
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- $ qemu-7.2 -M pc-7.2 -> qemu-8.0 -M pc-7.2
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- $ qemu-8.0 -M pc-7.2 -> qemu-7.2 -M pc-7.2
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This two are the ones that work. The whole point of making the
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change in qemu-8.0.1 release was to fix this issue:
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- $ qemu-7.2 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2
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- $ qemu-8.0.1 -M pc-7.2 -> qemu-7.2 -M pc-7.2
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But now we found that qemu-8.0 neither can migrate to qemu-7.2 not
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qemu-8.0.1.
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- $ qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2
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- $ qemu-8.0.1 -M pc-7.2 -> qemu-8.0 -M pc-7.2
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So, if we start a pc-7.2 machine in qemu-8.0 we can't migrate it to
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anything except to qemu-8.0.
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Can we do better?
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Yeap. If we know that we are going to do this migration:
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- $ qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2
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We can launch the appropriate devices with::
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--device...,x-pci-e-err-unc-mask=on
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And now we can receive a migration from 8.0. And from now on, we can
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do that migration to new machine types if we remember to enable that
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property for pc-7.2. Notice that we need to remember, it is not
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enough to know that the source of the migration is qemu-8.0. Think of
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this example:
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$ qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 -> qemu-8.2 -M pc-7.2
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In the second migration, the source is not qemu-8.0, but we still have
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that "problem" and have that property enabled. Notice that we need to
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continue having this mark/property until we have this machine
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rebooted. But it is not a normal reboot (that don't reload QEMU) we
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need the machine to poweroff/poweron on a fixed QEMU. And from now
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on we can use the proper real machine.
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