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
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
|
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
::
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)
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