mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
synced 2026-07-24 23:13:37 -04:00
d7140b5dde
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 as 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. We come to solve the case when a new chunk of critical data is introduced during the PRE_COPY phase and the driver would like to report an entirely new value for the initial_bytes. 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. 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 by enabling the VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2 device feature. When the caller opts in, the driver may report an entirely new value for initial_bytes. It may be larger, it may be smaller, it may include the previous unread initial_bytes, it may discard the previous unread initial_bytes, up to the driver logic and state. The presence of the VFIO_PRECOPY_INFO_REINIT output flag set by the driver indicates that new initial data is present on the stream. Once the caller sees this flag, the initial_bytes value should be re-evaluated relative to the readiness state for transition to STOP_COPY. Signed-off-by: Yishai Hadas <yishaih@nvidia.com> Link: https://lore.kernel.org/r/20260317161753.18964-2-yishaih@nvidia.com Signed-off-by: Alex Williamson <alex@shazbot.org>