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Document interfaces used for VFIO device migration. Added flow of state changes during live migration with VFIO device. Reviewed-by: Cornelia Huck <cohuck@redhat.com> Co-developed-by: Kirti Wankhede <kwankhede@nvidia.com> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com> Signed-off-by: Tarun Gupta <targupta@nvidia.com> Message-Id: <20210418122251.88809-1-targupta@nvidia.com> Signed-off-by: Alex Williamson <alex.williamson@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 can choose to skip
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the pre-copy phase of migration by returning pending_bytes as zero during the
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pre-copy phase.
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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_migration_info`` structure in the header
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file 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 the migration region and sets _SAVING
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flag in the VFIO device state.
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* A ``load_setup`` function that sets up the migration region on the
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destination and sets _RESUMING flag in the VFIO device state.
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* A ``save_live_pending`` 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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* A ``save_live_iterate`` function that reads the VFIO device's data from the
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vendor driver through the migration region during iterative phase.
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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 resets _RUNNING flag from the
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VFIO device state and iteratively copies the remaining data for the VFIO
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device until the vendor driver indicates that no data remains (pending bytes
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is zero).
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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, including unmapping the migration region
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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 IOMMU module to start and stop dirty page tracking. A ``log_sync``
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memory listener callback marks those system memory pages as dirty which are
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used for DMA by the VFIO device. The dirty pages bitmap is queried per
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container. All pages pinned by the vendor driver through external APIs have to
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be marked as dirty during migration. When there are CPU writes, CPU dirty page
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tracking can identify dirtied pages, but any page pinned by the vendor driver
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can also be written by the device. There is currently no device or IOMMU
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support for dirty page tracking in hardware.
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By default, dirty pages are tracked when the device is in pre-copy as well as
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stop-and-copy phase. So, a page pinned by the vendor driver will be copied to
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the destination in both phases. Copying dirty pages in pre-copy phase helps
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QEMU to predict if it can achieve its downtime tolerances. If QEMU during
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pre-copy phase keeps finding dirty pages continuously, then it understands
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that even in stop-and-copy phase, it is likely to find dirty pages and can
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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.
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Flow of state changes during Live migration
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===========================================
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Below is the flow of state change during live migration.
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The values in the brackets 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, _RUNNING|_SAVING)
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(RUNNING, _ACTIVE, _RUNNING|_SAVING)
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If device is active, get pending_bytes by .save_live_pending()
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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, vCPU stops and calls .save_live_complete_precopy for
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each active device. The VFIO device is then transitioned into _SAVING state
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(FINISH_MIGRATE, _DEVICE, _SAVING)
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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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(FINISH_MIGRATE, _DEVICE, _STOPPED)
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(FINISH_MIGRATE, _COMPLETED, _STOPPED)
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Migraton thread schedules cleanup bottom half and exits
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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, _STOPPED)
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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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(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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