333f988d19
Adjust the VFIO dirty page tracking documentation and add a section to describe device dirty page tracking. Signed-off-by: Avihai Horon <avihaih@nvidia.com> Signed-off-by: Joao Martins <joao.m.martins@oracle.com> Reviewed-by: Cédric Le Goater <clg@redhat.com> Link: https://lore.kernel.org/r/20230307125450.62409-16-joao.m.martins@oracle.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 currently consists of a single stop-and-copy phase.
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During the stop-and-copy phase the guest is stopped and the entire VFIO device
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data is transferred to the destination.
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The pre-copy phase of migration is currently not supported for VFIO devices.
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Support for VFIO pre-copy will be added later on.
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Note that currently VFIO migration is supported only for a single device. This
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is due to VFIO migration's lack of P2P support. However, P2P support is planned
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to be added later on.
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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_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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* 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 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)
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(RUNNING, _ACTIVE, _RUNNING)
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If device is active, get pending_bytes by .state_pending_exact()
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If total pending_bytes >= threshold_size, call .save_live_iterate()
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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 _STOP_COPY state
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(FINISH_MIGRATE, _DEVICE, _STOP_COPY)
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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, _STOP)
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(FINISH_MIGRATE, _COMPLETED, _STOP)
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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, _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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(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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