[PATCH for-8.2 v3 0/6] vfio/migration: Add P2P support for VFIO migration

[Avihai Horon]

Hi all,

The first patch in this series adds a small optimization to VFIO
migration by moving the STOP_COPY->STOP transition to
vfio_save_cleanup(). Testing with a ConnectX-7 VFIO device showed that
this can reduce downtime by up to 6%.

The rest of the series adds P2P support for VFIO migration.

VFIO migration uAPI defines an optional intermediate P2P quiescent
state. While in the P2P quiescent state, P2P DMA transactions cannot be
initiated by the device, but the device can respond to incoming ones.
Additionally, all outstanding P2P transactions are guaranteed to have
been completed by the time the device enters this state.

The purpose of this state is to support migration of multiple devices
that might do P2P transactions between themselves.

To implement P2P migration support, all the devices must be transitioned
to the P2P quiescent state before being stopped or started.

This behavior is achieved by adding an optional prepare callback to
VMChangeStateEntry. These callbacks are invoked before the main VM state
change callbacks, transitioning all the VFIO devices to the P2P state,
and only then are the main callbacks invoked, which stop or start the
devices.

This will allow migration of multiple VFIO devices if all of them
support P2P migration.

Thanks.

Changes from v2 [2]:
* Registered VFIO prepare callback only if P2P migration is supported by
  the device. (Cedric)
* Added Cedric R-b tags to patches 2,3 and 6.

Changes from v1 [1]:
* Rebased on latest master branch.
* Renamed pre_change_cb to prepare_cb and adjusted relevant code
  accordingly. (Cedric)
* Split VFIO vmstate change handler to two separate handlers, one for
  prepare callback and another for main callback. (Cedric)
* Renamed vfio_should_block_multiple_devices_migration() to
  vfio_multiple_devices_migration_is_supported() and reverted its logic
  accordingly. (Cedric)
* Rephrased "that are doing P2P" to "that might do P2P" in docs and
  commit message. (Jason)
* Added Cedric R-b tag to patch #4.

[1]
https://lore.kernel.org/qemu-devel/20230716081541.27900-1-avihaih@nvidia.com/

[2]
https://lore.kernel.org/qemu-devel/20230731102521.15335-1-avihaih@nvidia.com/

Avihai Horon (5):
  vfio/migration: Move from STOP_COPY to STOP in vfio_save_cleanup()
  sysemu: Add prepare callback to struct VMChangeStateEntry
  qdev: Add qdev_add_vm_change_state_handler_full()
  vfio/migration: Add P2P support for VFIO migration
  vfio/migration: Allow migration of multiple P2P supporting devices

Joao Martins (1):
  vfio/migration: Refactor PRE_COPY and RUNNING state checks

 docs/devel/vfio-migration.rst     | 93 +++++++++++++++++++------------
 include/hw/vfio/vfio-common.h     |  2 +
 include/sysemu/runstate.h         |  7 +++
 hw/core/vm-change-state-handler.c | 14 ++++-
 hw/vfio/common.c                  | 50 +++++++++++++----
 hw/vfio/migration.c               | 75 ++++++++++++++++++++-----
 softmmu/runstate.c                | 40 +++++++++++++
 hw/vfio/trace-events              |  1 +
 8 files changed, 218 insertions(+), 64 deletions(-)

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2.26.3