f6bbac985e
Create an index page for migration module. Move VFIO migration there too. A trivial touch-up on the title to use lower case there. Since then we'll have "migration" as the top title, make the main doc file renamed to "migration framework". Cc: Alex Williamson <alex.williamson@redhat.com> Cc: Cédric Le Goater <clg@redhat.com> Reviewed-by: Cédric Le Goater <clg@redhat.com> Link: https://lore.kernel.org/r/20240109064628.595453-3-peterx@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com>
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=====================
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VFIO device migration
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=====================
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Migration of virtual machine involves saving the state for each device that
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the guest is running on source host and restoring this saved state on the
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destination host. This document details how saving and restoring of VFIO
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devices is done in QEMU.
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Migration of VFIO devices consists of two phases: the optional pre-copy phase,
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and the stop-and-copy phase. The pre-copy phase is iterative and allows to
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accommodate VFIO devices that have a large amount of data that needs to be
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transferred. The iterative pre-copy phase of migration allows for the guest to
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continue whilst the VFIO device state is transferred to the destination, this
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helps to reduce the total downtime of the VM. VFIO devices opt-in to pre-copy
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support by reporting the VFIO_MIGRATION_PRE_COPY flag in the
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VFIO_DEVICE_FEATURE_MIGRATION ioctl.
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When pre-copy is supported, it's possible to further reduce downtime by
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enabling "switchover-ack" migration capability.
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VFIO migration uAPI defines "initial bytes" as part of its pre-copy data stream
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and recommends that the initial bytes are sent and loaded in the destination
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before stopping the source VM. Enabling this migration capability will
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guarantee that and thus, can potentially reduce downtime even further.
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To support migration of multiple devices that might do P2P transactions between
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themselves, VFIO migration uAPI defines an intermediate P2P quiescent state.
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While in the P2P quiescent state, P2P DMA transactions cannot be initiated by
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the device, but the device can respond to incoming ones. Additionally, all
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outstanding P2P transactions are guaranteed to have been completed by the time
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the device enters this state.
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All the devices that support P2P migration are first transitioned to the P2P
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quiescent state and only then are they stopped or started. This makes migration
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safe P2P-wise, since starting and stopping the devices is not done atomically
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for all the devices together.
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Thus, multiple VFIO devices migration is allowed only if all the devices
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support P2P migration. Single VFIO device migration is allowed regardless of
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P2P migration support.
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A detailed description of the UAPI for VFIO device migration can be found in
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the comment for the ``vfio_device_mig_state`` structure in the header file
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linux-headers/linux/vfio.h.
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VFIO implements the device hooks for the iterative approach as follows:
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* A ``save_setup`` function that sets up migration on the source.
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* A ``load_setup`` function that sets the VFIO device on the destination in
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_RESUMING state.
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* A ``state_pending_estimate`` function that reports an estimate of the
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remaining pre-copy data that the vendor driver has yet to save for the VFIO
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device.
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* A ``state_pending_exact`` function that reads pending_bytes from the vendor
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driver, which indicates the amount of data that the vendor driver has yet to
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save for the VFIO device.
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* An ``is_active_iterate`` function that indicates ``save_live_iterate`` is
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active only when the VFIO device is in pre-copy states.
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* A ``save_live_iterate`` function that reads the VFIO device's data from the
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vendor driver during iterative pre-copy phase.
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* A ``switchover_ack_needed`` function that checks if the VFIO device uses
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"switchover-ack" migration capability when this capability is enabled.
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* A ``save_state`` function to save the device config space if it is present.
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* A ``save_live_complete_precopy`` function that sets the VFIO device in
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_STOP_COPY state and iteratively copies the data for the VFIO device until
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the vendor driver indicates that no data remains.
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* A ``load_state`` function that loads the config section and the data
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sections that are generated by the save functions above.
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* ``cleanup`` functions for both save and load that perform any migration
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related cleanup.
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The VFIO migration code uses a VM state change handler to change the VFIO
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device state when the VM state changes from running to not-running, and
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vice versa.
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Similarly, a migration state change handler is used to trigger a transition of
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the VFIO device state when certain changes of the migration state occur. For
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example, the VFIO device state is transitioned back to _RUNNING in case a
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migration failed or was canceled.
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System memory dirty pages tracking
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----------------------------------
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A ``log_global_start`` and ``log_global_stop`` memory listener callback informs
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the VFIO dirty tracking module to start and stop dirty page tracking. A
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``log_sync`` memory listener callback queries the dirty page bitmap from the
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dirty tracking module and marks system memory pages which were DMA-ed by the
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VFIO device as dirty. The dirty page bitmap is queried per container.
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Currently there are two ways dirty page tracking can be done:
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(1) Device dirty tracking:
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In this method the device is responsible to log and report its DMAs. This
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method can be used only if the device is capable of tracking its DMAs.
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Discovering device capability, starting and stopping dirty tracking, and
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syncing the dirty bitmaps from the device are done using the DMA logging uAPI.
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More info about the uAPI can be found in the comments of the
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``vfio_device_feature_dma_logging_control`` and
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``vfio_device_feature_dma_logging_report`` structures in the header file
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linux-headers/linux/vfio.h.
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(2) VFIO IOMMU module:
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In this method dirty tracking is done by IOMMU. However, there is currently no
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IOMMU support for dirty page tracking. For this reason, all pages are
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perpetually marked dirty, unless the device driver pins pages through external
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APIs in which case only those pinned pages are perpetually marked dirty.
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If the above two methods are not supported, all pages are perpetually marked
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dirty by QEMU.
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By default, dirty pages are tracked during pre-copy as well as stop-and-copy
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phase. So, a page marked as dirty will be copied to the destination in both
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phases. Copying dirty pages in pre-copy phase helps QEMU to predict if it can
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achieve its downtime tolerances. If QEMU during pre-copy phase keeps finding
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dirty pages continuously, then it understands that even in stop-and-copy phase,
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it is likely to find dirty pages and can predict the downtime accordingly.
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QEMU also provides a per device opt-out option ``pre-copy-dirty-page-tracking``
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which disables querying the dirty bitmap during pre-copy phase. If it is set to
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off, all dirty pages will be copied to the destination in stop-and-copy phase
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only.
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System memory dirty pages tracking when vIOMMU is enabled
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---------------------------------------------------------
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With vIOMMU, an IO virtual address range can get unmapped while in pre-copy
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phase of migration. In that case, the unmap ioctl returns any dirty pages in
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that range and QEMU reports corresponding guest physical pages dirty. During
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stop-and-copy phase, an IOMMU notifier is used to get a callback for mapped
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pages and then dirty pages bitmap is fetched from VFIO IOMMU modules for those
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mapped ranges. If device dirty tracking is enabled with vIOMMU, live migration
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will be blocked.
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Flow of state changes during Live migration
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===========================================
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Below is the state change flow during live migration for a VFIO device that
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supports both precopy and P2P migration. The flow for devices that don't
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support it is similar, except that the relevant states for precopy and P2P are
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skipped.
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The values in the parentheses represent the VM state, the migration state, and
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the VFIO device state, respectively.
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Live migration save path
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------------------------
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::
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QEMU normal running state
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(RUNNING, _NONE, _RUNNING)
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migrate_init spawns migration_thread
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Migration thread then calls each device's .save_setup()
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(RUNNING, _SETUP, _PRE_COPY)
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(RUNNING, _ACTIVE, _PRE_COPY)
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If device is active, get pending_bytes by .state_pending_{estimate,exact}()
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If total pending_bytes >= threshold_size, call .save_live_iterate()
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Data of VFIO device for pre-copy phase is copied
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Iterate till total pending bytes converge and are less than threshold
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On migration completion, the vCPUs and the VFIO device are stopped
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The VFIO device is first put in P2P quiescent state
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(FINISH_MIGRATE, _ACTIVE, _PRE_COPY_P2P)
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Then the VFIO device is put in _STOP_COPY state
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(FINISH_MIGRATE, _ACTIVE, _STOP_COPY)
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.save_live_complete_precopy() is called for each active device
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For the VFIO device, iterate in .save_live_complete_precopy() until
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pending data is 0
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(POSTMIGRATE, _COMPLETED, _STOP_COPY)
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Migraton thread schedules cleanup bottom half and exits
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.save_cleanup() is called
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(POSTMIGRATE, _COMPLETED, _STOP)
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Live migration resume path
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--------------------------
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::
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Incoming migration calls .load_setup() for each device
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(RESTORE_VM, _ACTIVE, _STOP)
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For each device, .load_state() is called for that device section data
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(RESTORE_VM, _ACTIVE, _RESUMING)
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At the end, .load_cleanup() is called for each device and vCPUs are started
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The VFIO device is first put in P2P quiescent state
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(RUNNING, _ACTIVE, _RUNNING_P2P)
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(RUNNING, _NONE, _RUNNING)
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Postcopy
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========
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Postcopy migration is currently not supported for VFIO devices.
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