Re: [PATCH V2 vfio 0/6] Add support for PRE_COPY initial bytes re-initialization

[Alex Williamson <alex@shazbot.org>]

On Tue, 17 Mar 2026 18:17:47 +0200
Yishai Hadas <yishaih@nvidia.com> wrote:

> This series introduces support for re-initializing the initial_bytes
> value during the VFIO PRE_COPY migration phase.
> 
> Background
> ==========
> As currently defined, initial_bytes is monotonically decreasing and
> precedes dirty_bytes when reading from the saving file descriptor.
> The transition from initial_bytes to dirty_bytes is unidirectional and
> irreversible.
> 
> The initial_bytes are considered critical data that is highly
> recommended to be transferred to the target as part of PRE_COPY.
> Without this data, the PRE_COPY phase would be ineffective.
> 
> Problem Statement
> =================
> In some cases, a new chunk of critical data may appear during the
> PRE_COPY phase. The current API does not provide a mechanism for the
> driver to report an updated initial_bytes value when this occurs.
> 
> Solution
> ========
> For that, we extend the VFIO_MIG_GET_PRECOPY_INFO ioctl with an output
> flag named VFIO_PRECOPY_INFO_REINIT to allow drivers reporting a new
> initial_bytes value during the PRE_COPY phase.
> 
> However, Currently, existing VFIO_MIG_GET_PRECOPY_INFO implementations
> don't assign info.flags before copy_to_user(), this effectively echoes
> userspace-provided flags back as output, preventing the field from being
> used to report new reliable data from the drivers.
> 
> Reliable use of the new VFIO_PRECOPY_INFO_REINIT flag requires userspace
> to explicitly opt in. For that we introduce a new feature named
> VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2.
> 
> User should opt-in to the above feature with a SET operation, no data is
> required and any supplied data is ignored.
> 
> When the caller opts in:
> - We set info.flags to zero, otherwise we keep v1 behaviour as is for
>   compatibility reasons.
> - The new output flag VFIO_PRECOPY_INFO_REINIT can be used reliably.
> - The VFIO_PRECOPY_INFO_REINIT output flag indicates that new initial
>   data is present on the stream. The initial_bytes value should be
>   re-evaluated relative to the readiness state for transition to
>   STOP_COPY.
> 
> The mlx5 VFIO driver is extended to support this case when the
> underlying firmware also supports the REINIT migration state.
> 
> As part of this series, a core helper function is introduced to provide
> shared functionality for implementing the VFIO_MIG_GET_PRECOPY_INFO
> ioctl, and all drivers have been updated to use it.
> 
> Changes from V1:
> https://patchwork.kernel.org/project/kvm/cover/20260310164006.4020-1-yishaih@nvidia.com/
> 
> Patch #1:
> - Extend the uAPI documentation to refer to the source of new
>   initial_bytes data.

Applied with the fix discussed in 2/ to vfio next branch for v7.1.
Thanks,

Alex