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 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_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_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_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 brackets represent the VM state, the migration state, and the VFIO device state, respectively.

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)
                    (RUNNING, _ACTIVE, _RUNNING)
           If device is active, get pending_bytes by .state_pending_exact()
        If total pending_bytes >= threshold_size, call .save_live_iterate()
      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
   For the VFIO device, iterate in .save_live_complete_precopy until
                       pending data is 0
                 (FINISH_MIGRATE, _DEVICE, _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 migration is currently not supported for VFIO devices.