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vfio/migration: Implement VFIO_PRECOPY_INFO_REINIT feature

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According to VFIO uAPI, precopy initial_bytes is considered as critical
data that should be transferred and loaded prior to moving to STOP_COPY
state to ensure precopy phase would be effective.

As currently defined, initial_bytes can only decrease as it's being read
from the data fd. However, there are cases where a new chunk of
initial_bytes should be transferred during precopy.

The new VFIO_PRECOPY_INFO_REINIT feature addresses this and allows
reporting a new value for initial_bytes regardless of any previously
reported values.

Implement VFIO_PRECOPY_INFO_REINIT feature:
1. Opt-in for VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2 to make
   VFIO_PRECOPY_INFO_REINIT available.
2. Request a new switchover ACK if initial_bytes increases post of a
   previous switchover ACK. This ensures the device is not moved to
   STOP_COPY before initial_bytes has reached zero again.

Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Link: https://lore.kernel.org/qemu-devel/20260706085211.13905-13-avihaih@nvidia.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
This commit is contained in:
Avihai Horon
2026-07-06 11:52:07 +03:00
committed by Cédric Le Goater
parent f4ba2df1d9
commit 5fcadd7d14
4 changed files with 80 additions and 7 deletions
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14
docs/devel/migration/vfio.rst
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@@ -23,6 +23,20 @@ and recommends that the initial bytes are sent and loaded in the destination
before stopping the source VM. Enabling this migration capability will
guarantee that and thus, can potentially reduce downtime even further.
For example, in mlx5 devices, the initial bytes hold metadata used for time
consuming pre-allocations of resources on the destination. Although init bytes
may be small in size and sending them may take little time, loading them in the
destination can take a significant amount of time. Switchover-ack guarantees
that this pre-allocation doesn't happen during downtime.
Initial bytes was originally defined to be monotonically decreasing, however
there are cases where a new chunk of initial bytes should be transferred during
precopy, e.g., due to a device reconfiguration, etc. The
VFIO_PRECOPY_INFO_REINIT feature addresses this and when supported, allows to
report a new initial bytes value regardless of any previously reported values.
In this case, a new switchover ACK will be requested to make sure the new
initial bytes are loaded in the destination before switching over.
To support migration of multiple devices that might do P2P transactions between
themselves, VFIO migration uAPI defines an intermediate P2P quiescent state.
While in the P2P quiescent state, P2P DMA transactions cannot be initiated by