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=====================
VFIO device Migration
=====================
Migration of virtual machine involves saving the state for each device that
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 opt-in to pre-copy
support by reporting the VFIO_MIGRATION_PRE_COPY flag in the
VFIO_DEVICE_FEATURE_MIGRATION ioctl.
When pre-copy is supported, it's possible to further reduce downtime by
enabling "switchover-ack" migration capability.
VFIO migration uAPI defines "initial bytes" as part of its pre-copy data stream
and recommends that the initial bytes are sent and loaded in the destination
before stopping the source VM. Enabling this migration capability will
guarantee that and thus, can potentially reduce downtime even further.
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_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 migration on the source.
* A ``load_setup`` function that sets the VFIO device on the destination in
_RESUMING state.
* A ``state_pending_estimate`` function that reports an estimate of the
remaining pre-copy data that the vendor driver has yet to save for the VFIO
device.
* 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.
* An ``is_active_iterate`` function that indicates ``save_live_iterate`` is
active only when the VFIO device is in pre-copy states.
* A ``save_live_iterate`` function that reads the VFIO device's data from the
vendor driver during iterative pre-copy phase.
* A ``switchover_ack_needed`` function that checks if the VFIO device uses
"switchover-ack" migration capability when this capability is enabled.
* A ``save_state`` function to save the device config space if it is present.
* 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.
* ``cleanup`` functions for both save and load that perform any migration
related cleanup.
The VFIO migration code uses a VM state change handler to change the VFIO
device state when the VM state changes from running to not-running, and
vice versa.
Similarly, a migration state change handler is used to trigger a transition of
the VFIO device state when certain changes of the migration state occur. For
example, the VFIO device state is transitioned back to _RUNNING in case a
migration failed or was canceled.
System memory dirty pages tracking
----------------------------------
A ``log_global_start`` and ``log_global_stop`` memory listener callback informs
the VFIO dirty tracking module to start and stop dirty page tracking. A
``log_sync`` memory listener callback queries the dirty page bitmap from the
dirty tracking module and marks system memory pages which were DMA-ed by the
VFIO device as dirty. The dirty page bitmap is queried per container.
Currently there are two ways dirty page tracking can be done:
(1) Device dirty tracking:
In this method the device is responsible to log and report its DMAs. This
method can be used only if the device is capable of tracking its DMAs.
Discovering device capability, starting and stopping dirty tracking, and
syncing the dirty bitmaps from the device are done using the DMA logging uAPI.
More info about the uAPI can be found in the comments of the
``vfio_device_feature_dma_logging_control`` and
``vfio_device_feature_dma_logging_report`` structures in the header file
linux-headers/linux/vfio.h.
(2) VFIO IOMMU module:
In this method dirty tracking is done by IOMMU. However, there is currently no
IOMMU support for dirty page tracking. For this reason, all pages are
perpetually marked dirty, unless the device driver pins pages through external
APIs in which case only those pinned pages are perpetually marked dirty.
If the above two methods are not supported, all pages are perpetually marked
dirty by QEMU.
By default, dirty pages are tracked during pre-copy as well as stop-and-copy
phase. So, a page marked as dirty 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
off, all dirty pages will be copied to the destination in stop-and-copy phase
only.
System memory dirty pages tracking when vIOMMU is enabled
---------------------------------------------------------
With vIOMMU, an IO virtual address range can get unmapped while in pre-copy
phase of migration. In that case, the unmap ioctl returns any dirty pages in
that range and QEMU reports corresponding guest physical pages dirty. During
stop-and-copy phase, an IOMMU notifier is used to get a callback for mapped
pages and then dirty pages bitmap is fetched from VFIO IOMMU modules for those
mapped ranges. If device dirty tracking is enabled with vIOMMU, live migration
will be blocked.
Flow of state changes during Live migration
===========================================
Below is the flow of state change during live migration.
The values in the parentheses represent the VM state, the migration state, and
the VFIO device state, respectively.
The text in the square brackets represents the flow if the VFIO device supports
pre-copy.
Live migration save path
------------------------
::
QEMU normal running state
(RUNNING, _NONE, _RUNNING)
|
migrate_init spawns migration_thread
Migration thread then calls each device's .save_setup()
(RUNNING, _SETUP, _RUNNING [_PRE_COPY])
|
(RUNNING, _ACTIVE, _RUNNING [_PRE_COPY])
If device is active, get pending_bytes by .state_pending_{estimate,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 _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, _STOP)
|
(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, _STOP)
|
For each device, .load_state is called for that device section data
(RESTORE_VM, _ACTIVE, _RESUMING)
|
At the end, .load_cleanup is called for each device and vCPUs are started
(RUNNING, _NONE, _RUNNING)
Postcopy
========
Postcopy migration is currently not supported for VFIO devices.
|
