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docs/devel: Align VFIO migration docs to v2 protocol 

Now that VFIO migration protocol v2 has been implemented and v1 protocol
has been removed, update the documentation according to v2 protocol.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Link: https://lore.kernel.org/r/20230216143630.25610-12-avihaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
This commit is contained in:
Avihai Horon 2023-02-16 16:36:30 +02:00 committed by Alex Williamson
parent 48e4d8289f
commit 2b0ab9e9d6
1 changed files with 34 additions and 38 deletions
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72
docs/devel/vfio-migration.rst
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@ -7,46 +7,43 @@ the guest is running on source host and restoring this saved state on the
destination host. This document details how saving and restoring of VFIO
devices is done in QEMU.
Migration of VFIO devices consists of two phases: the optional pre-copy phase,
and the stop-and-copy phase. The pre-copy phase is iterative and allows to
accommodate VFIO devices that have a large amount of data that needs to be
transferred. The iterative pre-copy phase of migration allows for the guest to
continue whilst the VFIO device state is transferred to the destination, this
helps to reduce the total downtime of the VM. VFIO devices can choose to skip
the pre-copy phase of migration by returning pending_bytes as zero during the
pre-copy phase.
Migration of VFIO devices currently consists of a single stop-and-copy phase.
During the stop-and-copy phase the guest is stopped and the entire VFIO device
data is transferred to the destination.
The pre-copy phase of migration is currently not supported for VFIO devices.
Support for VFIO pre-copy will be added later on.
Note that currently VFIO migration is supported only for a single device. This
is due to VFIO migration's lack of P2P support. However, P2P support is planned
to be added later on.
A detailed description of the UAPI for VFIO device migration can be found in
the comment for the ``vfio_device_migration_info`` structure in the header
file linux-headers/linux/vfio.h.
the comment for the ``vfio_device_mig_state`` structure in the header file
linux-headers/linux/vfio.h.
VFIO implements the device hooks for the iterative approach as follows:
* A ``save_setup`` function that sets up the migration region and sets _SAVING
flag in the VFIO device state.
* A ``save_setup`` function that sets up migration on the source.
* A ``load_setup`` function that sets up the migration region on the
destination and sets _RESUMING flag in the VFIO device state.
* A ``load_setup`` function that sets the VFIO device on the destination in
_RESUMING state.
* A ``state_pending_exact`` function that reads pending_bytes from the vendor
driver, which indicates the amount of data that the vendor driver has yet to
save for the VFIO device.
* A ``save_live_iterate`` function that reads the VFIO device's data from the
vendor driver through the migration region during iterative phase.
* A ``save_state`` function to save the device config space if it is present.
* A ``save_live_complete_precopy`` function that resets _RUNNING flag from the
VFIO device state and iteratively copies the remaining data for the VFIO
device until the vendor driver indicates that no data remains (pending bytes
is zero).
* A ``save_live_complete_precopy`` function that sets the VFIO device in
_STOP_COPY state and iteratively copies the data for the VFIO device until
the vendor driver indicates that no data remains.
* A ``load_state`` function that loads the config section and the data
sections that are generated by the save functions above
sections that are generated by the save functions above.
* ``cleanup`` functions for both save and load that perform any migration
related cleanup, including unmapping the migration region
related cleanup.
The VFIO migration code uses a VM state change handler to change the VFIO
@ -71,13 +68,13 @@ tracking can identify dirtied pages, but any page pinned by the vendor driver
can also be written by the device. There is currently no device or IOMMU
support for dirty page tracking in hardware.
By default, dirty pages are tracked when the device is in pre-copy as well as
stop-and-copy phase. So, a page pinned by the vendor driver will be copied to
the destination in both phases. Copying dirty pages in pre-copy phase helps
QEMU to predict if it can achieve its downtime tolerances. If QEMU during
pre-copy phase keeps finding dirty pages continuously, then it understands
that even in stop-and-copy phase, it is likely to find dirty pages and can
predict the downtime accordingly.
By default, dirty pages are tracked during pre-copy as well as stop-and-copy
phase. So, a page pinned by the vendor driver will be copied to the destination
in both phases. Copying dirty pages in pre-copy phase helps QEMU to predict if
it can achieve its downtime tolerances. If QEMU during pre-copy phase keeps
finding dirty pages continuously, then it understands that even in stop-and-copy
phase, it is likely to find dirty pages and can predict the downtime
accordingly.
QEMU also provides a per device opt-out option ``pre-copy-dirty-page-tracking``
which disables querying the dirty bitmap during pre-copy phase. If it is set to
@ -111,23 +108,22 @@ Live migration save path
|
migrate_init spawns migration_thread
Migration thread then calls each device's .save_setup()
(RUNNING, _SETUP, _RUNNING|_SAVING)
(RUNNING, _SETUP, _RUNNING)
|
(RUNNING, _ACTIVE, _RUNNING|_SAVING)
(RUNNING, _ACTIVE, _RUNNING)
If device is active, get pending_bytes by .state_pending_exact()
If total pending_bytes >= threshold_size, call .save_live_iterate()
Data of VFIO device for pre-copy phase is copied
Iterate till total pending bytes converge and are less than threshold
|
On migration completion, vCPU stops and calls .save_live_complete_precopy for
each active device. The VFIO device is then transitioned into _SAVING state
(FINISH_MIGRATE, _DEVICE, _SAVING)
each active device. The VFIO device is then transitioned into _STOP_COPY state
(FINISH_MIGRATE, _DEVICE, _STOP_COPY)
|
For the VFIO device, iterate in .save_live_complete_precopy until
pending data is 0
(FINISH_MIGRATE, _DEVICE, _STOPPED)
(FINISH_MIGRATE, _DEVICE, _STOP)
|
(FINISH_MIGRATE, _COMPLETED, _STOPPED)
(FINISH_MIGRATE, _COMPLETED, _STOP)
Migraton thread schedules cleanup bottom half and exits
Live migration resume path
@ -136,7 +132,7 @@ Live migration resume path
::
Incoming migration calls .load_setup for each device
(RESTORE_VM, _ACTIVE, _STOPPED)
(RESTORE_VM, _ACTIVE, _STOP)
|
For each device, .load_state is called for that device section data
(RESTORE_VM, _ACTIVE, _RESUMING)