Re: [PATCH vfio 1/6] vfio: Define uAPI for re-init initial bytes during the PRE_COPY phase

[Alex Williamson <alex@shazbot.org>]

On Tue, 24 Feb 2026 10:20:14 +0200
Yishai Hadas <yishaih@nvidia.com> wrote:

> 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>
> ---
>  include/uapi/linux/vfio.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index bb7b89330d35..b6efda07000f 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -1266,6 +1266,17 @@ enum vfio_device_mig_state {
>   * The initial_bytes field indicates the amount of initial precopy
>   * data available from the device. This field should have a non-zero initial
>   * value and decrease as migration data is read from the device.
> + * The presence of the VFIO_PRECOPY_INFO_REINIT output flag indicates
> + * that new initial data is present on the stream.
> + * In that case initial_bytes may report a non-zero value irrespective of
> + * any previously reported values, which progresses towards zero as precopy
> + * data is read from the data stream. dirty_bytes is also reset
> + * to zero and represents the state change of the device relative to the new
> + * initial_bytes.
> + * VFIO_PRECOPY_INFO_REINIT can be reported only after userspace opts in to
> + * VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2. Without this opt-in, the flags field
> + * of struct vfio_precopy_info is reserved for bug-compatibility reasons.
> + *
>   * It is recommended to leave PRE_COPY for STOP_COPY only after this field
>   * reaches zero. Leaving PRE_COPY earlier might make things slower.
>   *
> @@ -1301,6 +1312,7 @@ enum vfio_device_mig_state {
>  struct vfio_precopy_info {
>  	__u32 argsz;
>  	__u32 flags;
> +#define VFIO_PRECOPY_INFO_REINIT (1 << 0) /* output - new initial data is present */
>  	__aligned_u64 initial_bytes;
>  	__aligned_u64 dirty_bytes;
>  };
> @@ -1510,6 +1522,16 @@ struct vfio_device_feature_dma_buf {
>  	struct vfio_region_dma_range dma_ranges[] __counted_by(nr_ranges);
>  };
>  
> +/*
> + * Enables the migration prepcopy_info_v2 behaviour.

s/prepcopy/precopy/

Thanks,
Alex


> + *
> + * VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2.
> + *
> + * On SET, enables the v2 pre_copy_info behaviour, where the
> + * vfio_precopy_info.flags is a valid output field.
> + */
> +#define VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2  12
> +
>  /* -------- API for Type1 VFIO IOMMU -------- */
>  
>  /**