485 lines
20 KiB
Plaintext
485 lines
20 KiB
Plaintext
= Migration =
|
|
|
|
QEMU has code to load/save the state of the guest that it is running.
|
|
These are two complementary operations. Saving the state just does
|
|
that, saves the state for each device that the guest is running.
|
|
Restoring a guest is just the opposite operation: we need to load the
|
|
state of each device.
|
|
|
|
For this to work, QEMU has to be launched with the same arguments the
|
|
two times. I.e. it can only restore the state in one guest that has
|
|
the same devices that the one it was saved (this last requirement can
|
|
be relaxed a bit, but for now we can consider that configuration has
|
|
to be exactly the same).
|
|
|
|
Once that we are able to save/restore a guest, a new functionality is
|
|
requested: migration. This means that QEMU is able to start in one
|
|
machine and being "migrated" to another machine. I.e. being moved to
|
|
another machine.
|
|
|
|
Next was the "live migration" functionality. This is important
|
|
because some guests run with a lot of state (specially RAM), and it
|
|
can take a while to move all state from one machine to another. Live
|
|
migration allows the guest to continue running while the state is
|
|
transferred. Only while the last part of the state is transferred has
|
|
the guest to be stopped. Typically the time that the guest is
|
|
unresponsive during live migration is the low hundred of milliseconds
|
|
(notice that this depends on a lot of things).
|
|
|
|
=== Types of migration ===
|
|
|
|
Now that we have talked about live migration, there are several ways
|
|
to do migration:
|
|
|
|
- tcp migration: do the migration using tcp sockets
|
|
- unix migration: do the migration using unix sockets
|
|
- exec migration: do the migration using the stdin/stdout through a process.
|
|
- fd migration: do the migration using an file descriptor that is
|
|
passed to QEMU. QEMU doesn't care how this file descriptor is opened.
|
|
|
|
All these four migration protocols use the same infrastructure to
|
|
save/restore state devices. This infrastructure is shared with the
|
|
savevm/loadvm functionality.
|
|
|
|
=== State Live Migration ===
|
|
|
|
This is used for RAM and block devices. It is not yet ported to vmstate.
|
|
<Fill more information here>
|
|
|
|
=== What is the common infrastructure ===
|
|
|
|
QEMU uses a QEMUFile abstraction to be able to do migration. Any type
|
|
of migration that wants to use QEMU infrastructure has to create a
|
|
QEMUFile with:
|
|
|
|
QEMUFile *qemu_fopen_ops(void *opaque,
|
|
QEMUFilePutBufferFunc *put_buffer,
|
|
QEMUFileGetBufferFunc *get_buffer,
|
|
QEMUFileCloseFunc *close);
|
|
|
|
The functions have the following functionality:
|
|
|
|
This function writes a chunk of data to a file at the given position.
|
|
The pos argument can be ignored if the file is only used for
|
|
streaming. The handler should try to write all of the data it can.
|
|
|
|
typedef int (QEMUFilePutBufferFunc)(void *opaque, const uint8_t *buf,
|
|
int64_t pos, int size);
|
|
|
|
Read a chunk of data from a file at the given position. The pos argument
|
|
can be ignored if the file is only be used for streaming. The number of
|
|
bytes actually read should be returned.
|
|
|
|
typedef int (QEMUFileGetBufferFunc)(void *opaque, uint8_t *buf,
|
|
int64_t pos, int size);
|
|
|
|
Close a file and return an error code.
|
|
|
|
typedef int (QEMUFileCloseFunc)(void *opaque);
|
|
|
|
You can use any internal state that you need using the opaque void *
|
|
pointer that is passed to all functions.
|
|
|
|
The important functions for us are put_buffer()/get_buffer() that
|
|
allow to write/read a buffer into the QEMUFile.
|
|
|
|
=== How to save the state of one device ===
|
|
|
|
The state of a device is saved using intermediate buffers. There are
|
|
some helper functions to assist this saving.
|
|
|
|
There is a new concept that we have to explain here: device state
|
|
version. When we migrate a device, we save/load the state as a series
|
|
of fields. Some times, due to bugs or new functionality, we need to
|
|
change the state to store more/different information. We use the
|
|
version to identify each time that we do a change. Each version is
|
|
associated with a series of fields saved. The save_state always saves
|
|
the state as the newer version. But load_state sometimes is able to
|
|
load state from an older version.
|
|
|
|
=== Legacy way ===
|
|
|
|
This way is going to disappear as soon as all current users are ported to VMSTATE.
|
|
|
|
Each device has to register two functions, one to save the state and
|
|
another to load the state back.
|
|
|
|
int register_savevm(DeviceState *dev,
|
|
const char *idstr,
|
|
int instance_id,
|
|
int version_id,
|
|
SaveStateHandler *save_state,
|
|
LoadStateHandler *load_state,
|
|
void *opaque);
|
|
|
|
typedef void SaveStateHandler(QEMUFile *f, void *opaque);
|
|
typedef int LoadStateHandler(QEMUFile *f, void *opaque, int version_id);
|
|
|
|
The important functions for the device state format are the save_state
|
|
and load_state. Notice that load_state receives a version_id
|
|
parameter to know what state format is receiving. save_state doesn't
|
|
have a version_id parameter because it always uses the latest version.
|
|
|
|
=== VMState ===
|
|
|
|
The legacy way of saving/loading state of the device had the problem
|
|
that we have to maintain two functions in sync. If we did one change
|
|
in one of them and not in the other, we would get a failed migration.
|
|
|
|
VMState changed the way that state is saved/loaded. Instead of using
|
|
a function to save the state and another to load it, it was changed to
|
|
a declarative way of what the state consisted of. Now VMState is able
|
|
to interpret that definition to be able to load/save the state. As
|
|
the state is declared only once, it can't go out of sync in the
|
|
save/load functions.
|
|
|
|
An example (from hw/input/pckbd.c)
|
|
|
|
static const VMStateDescription vmstate_kbd = {
|
|
.name = "pckbd",
|
|
.version_id = 3,
|
|
.minimum_version_id = 3,
|
|
.fields = (VMStateField[]) {
|
|
VMSTATE_UINT8(write_cmd, KBDState),
|
|
VMSTATE_UINT8(status, KBDState),
|
|
VMSTATE_UINT8(mode, KBDState),
|
|
VMSTATE_UINT8(pending, KBDState),
|
|
VMSTATE_END_OF_LIST()
|
|
}
|
|
};
|
|
|
|
We are declaring the state with name "pckbd".
|
|
The version_id is 3, and the fields are 4 uint8_t in a KBDState structure.
|
|
We registered this with:
|
|
|
|
vmstate_register(NULL, 0, &vmstate_kbd, s);
|
|
|
|
Note: talk about how vmstate <-> qdev interact, and what the instance ids mean.
|
|
|
|
You can search for VMSTATE_* macros for lots of types used in QEMU in
|
|
include/hw/hw.h.
|
|
|
|
=== More about versions ===
|
|
|
|
You can see that there are several version fields:
|
|
|
|
- version_id: the maximum version_id supported by VMState for that device.
|
|
- minimum_version_id: the minimum version_id that VMState is able to understand
|
|
for that device.
|
|
- minimum_version_id_old: For devices that were not able to port to vmstate, we can
|
|
assign a function that knows how to read this old state. This field is
|
|
ignored if there is no load_state_old handler.
|
|
|
|
So, VMState is able to read versions from minimum_version_id to
|
|
version_id. And the function load_state_old() (if present) is able to
|
|
load state from minimum_version_id_old to minimum_version_id. This
|
|
function is deprecated and will be removed when no more users are left.
|
|
|
|
=== Massaging functions ===
|
|
|
|
Sometimes, it is not enough to be able to save the state directly
|
|
from one structure, we need to fill the correct values there. One
|
|
example is when we are using kvm. Before saving the cpu state, we
|
|
need to ask kvm to copy to QEMU the state that it is using. And the
|
|
opposite when we are loading the state, we need a way to tell kvm to
|
|
load the state for the cpu that we have just loaded from the QEMUFile.
|
|
|
|
The functions to do that are inside a vmstate definition, and are called:
|
|
|
|
- int (*pre_load)(void *opaque);
|
|
|
|
This function is called before we load the state of one device.
|
|
|
|
- int (*post_load)(void *opaque, int version_id);
|
|
|
|
This function is called after we load the state of one device.
|
|
|
|
- void (*pre_save)(void *opaque);
|
|
|
|
This function is called before we save the state of one device.
|
|
|
|
Example: You can look at hpet.c, that uses the three function to
|
|
massage the state that is transferred.
|
|
|
|
If you use memory API functions that update memory layout outside
|
|
initialization (i.e., in response to a guest action), this is a strong
|
|
indication that you need to call these functions in a post_load callback.
|
|
Examples of such memory API functions are:
|
|
|
|
- memory_region_add_subregion()
|
|
- memory_region_del_subregion()
|
|
- memory_region_set_readonly()
|
|
- memory_region_set_enabled()
|
|
- memory_region_set_address()
|
|
- memory_region_set_alias_offset()
|
|
|
|
=== Subsections ===
|
|
|
|
The use of version_id allows to be able to migrate from older versions
|
|
to newer versions of a device. But not the other way around. This
|
|
makes very complicated to fix bugs in stable branches. If we need to
|
|
add anything to the state to fix a bug, we have to disable migration
|
|
to older versions that don't have that bug-fix (i.e. a new field).
|
|
|
|
But sometimes, that bug-fix is only needed sometimes, not always. For
|
|
instance, if the device is in the middle of a DMA operation, it is
|
|
using a specific functionality, ....
|
|
|
|
It is impossible to create a way to make migration from any version to
|
|
any other version to work. But we can do better than only allowing
|
|
migration from older versions to newer ones. For that fields that are
|
|
only needed sometimes, we add the idea of subsections. A subsection
|
|
is "like" a device vmstate, but with a particularity, it has a Boolean
|
|
function that tells if that values are needed to be sent or not. If
|
|
this functions returns false, the subsection is not sent.
|
|
|
|
On the receiving side, if we found a subsection for a device that we
|
|
don't understand, we just fail the migration. If we understand all
|
|
the subsections, then we load the state with success.
|
|
|
|
One important note is that the post_load() function is called "after"
|
|
loading all subsections, because a newer subsection could change same
|
|
value that it uses.
|
|
|
|
Example:
|
|
|
|
static bool ide_drive_pio_state_needed(void *opaque)
|
|
{
|
|
IDEState *s = opaque;
|
|
|
|
return ((s->status & DRQ_STAT) != 0)
|
|
|| (s->bus->error_status & BM_STATUS_PIO_RETRY);
|
|
}
|
|
|
|
const VMStateDescription vmstate_ide_drive_pio_state = {
|
|
.name = "ide_drive/pio_state",
|
|
.version_id = 1,
|
|
.minimum_version_id = 1,
|
|
.pre_save = ide_drive_pio_pre_save,
|
|
.post_load = ide_drive_pio_post_load,
|
|
.needed = ide_drive_pio_state_needed,
|
|
.fields = (VMStateField[]) {
|
|
VMSTATE_INT32(req_nb_sectors, IDEState),
|
|
VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
|
|
vmstate_info_uint8, uint8_t),
|
|
VMSTATE_INT32(cur_io_buffer_offset, IDEState),
|
|
VMSTATE_INT32(cur_io_buffer_len, IDEState),
|
|
VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
|
|
VMSTATE_INT32(elementary_transfer_size, IDEState),
|
|
VMSTATE_INT32(packet_transfer_size, IDEState),
|
|
VMSTATE_END_OF_LIST()
|
|
}
|
|
};
|
|
|
|
const VMStateDescription vmstate_ide_drive = {
|
|
.name = "ide_drive",
|
|
.version_id = 3,
|
|
.minimum_version_id = 0,
|
|
.post_load = ide_drive_post_load,
|
|
.fields = (VMStateField[]) {
|
|
.... several fields ....
|
|
VMSTATE_END_OF_LIST()
|
|
},
|
|
.subsections = (const VMStateDescription*[]) {
|
|
&vmstate_ide_drive_pio_state,
|
|
NULL
|
|
}
|
|
};
|
|
|
|
Here we have a subsection for the pio state. We only need to
|
|
save/send this state when we are in the middle of a pio operation
|
|
(that is what ide_drive_pio_state_needed() checks). If DRQ_STAT is
|
|
not enabled, the values on that fields are garbage and don't need to
|
|
be sent.
|
|
|
|
= Return path =
|
|
|
|
In most migration scenarios there is only a single data path that runs
|
|
from the source VM to the destination, typically along a single fd (although
|
|
possibly with another fd or similar for some fast way of throwing pages across).
|
|
|
|
However, some uses need two way communication; in particular the Postcopy
|
|
destination needs to be able to request pages on demand from the source.
|
|
|
|
For these scenarios there is a 'return path' from the destination to the source;
|
|
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)
|
|
|
|
= 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 or the network connection 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 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.
|
|
|
|
Note: During the postcopy phase, the bandwidth limits set using
|
|
migrate_set_speed 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 (into a QEMUSizedBuffer), 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 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).
|
|
|
|
End: The listen thread can now quit, and perform the cleanup of migration
|
|
state, the migration is now complete.
|
|
|
|
=== Source side page maps ===
|
|
|
|
The source side keeps two bitmaps during postcopy; 'the migration bitmap'
|
|
and 'unsent map'. The 'migration bitmap' is basically the same as in
|
|
the precopy case, and holds a 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.
|
|
|
|
The 'unsent map' is used for the transition to postcopy. It is a bitmap that
|
|
has a bit cleared whenever a page is sent to the destination, however during
|
|
the transition to postcopy mode it is combined with the migration bitmap
|
|
to form a set of pages that:
|
|
a) Have been sent but then redirtied (which must be discarded)
|
|
b) Have not yet been sent - which also must be discarded to cause any
|
|
transparent huge pages built during precopy to be broken.
|
|
|
|
Note that the contents of the unsentmap are sacrificed during the calculation
|
|
of the discard set and thus aren't valid once in postcopy. The dirtymap
|
|
is still valid and is used to ensure that no page is sent more than once. Any
|
|
request for a page that has already been sent is ignored. Duplicate requests
|
|
such as this can happen as a page is sent at about the same time the
|
|
destination accesses it.
|
|
|