RE: [PATCH V2 vfio 02/14] vfio: Extend the device migration protocol with PRE_COPY

[Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>]

> -----Original Message-----
> From: Yishai Hadas [mailto:yishaih@nvidia.com]
> Sent: 01 December 2022 15:29
> To: alex.williamson@redhat.com; jgg@nvidia.com
> Cc: kvm@vger.kernel.org; kevin.tian@intel.com; joao.m.martins@oracle.com;
> leonro@nvidia.com; shayd@nvidia.com; yishaih@nvidia.com;
> maorg@nvidia.com; avihaih@nvidia.com; cohuck@redhat.com
> Subject: [PATCH V2 vfio 02/14] vfio: Extend the device migration protocol
> with PRE_COPY

[...]
 
> +/**
> + * VFIO_MIG_GET_PRECOPY_INFO - _IO(VFIO_TYPE, VFIO_BASE + 21)
> + *
> + * This ioctl is used on the migration data FD in the precopy phase of the
> + * migration data transfer. It returns an estimate of the current data sizes
> + * remaining to be transferred. It allows the user to judge when it is
> + * appropriate to leave PRE_COPY for STOP_COPY.
> + *
> + * This ioctl is valid only in PRE_COPY states and kernel driver should
> + * return -EINVAL from any other migration state.
> + *
> + * The vfio_precopy_info data structure returned by this ioctl provides
> + * estimates of data available from the device during the PRE_COPY states.
> + * This estimate is split into two categories, initial_bytes and
> + * dirty_bytes.
> + *
> + * 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.
> + * It is recommended to leave PRE_COPY for STOP_COPY only after this field
> + * reaches zero. Leaving PRE_COPY earlier might make things slower.
> + *
> + * The dirty_bytes field tracks device state changes relative to data
> + * previously retrieved.  This field starts at zero and may increase as
> + * the internal device state is modified or decrease as that modified
> + * state is read from the device.
> + *
> + * Userspace may use the combination of these fields to estimate the
> + * potential data size available during the PRE_COPY phases, as well as
> + * trends relative to the rate the device is dirtying its internal
> + * state, but these fields are not required to have any bearing relative
> + * to the data size available during the STOP_COPY phase.
> + *
> + * Drivers have a lot of flexibility in when and what they transfer during the
> + * PRE_COPY phase, and how they report this from
> VFIO_MIG_GET_PRECOPY_INFO.
> + *
> + * During pre-copy the migration data FD has a temporary "end of stream"
> that is
> + * reached when both initial_bytes and dirty_byte are zero. For instance,
> this
> + * may indicate that the device is idle and not currently dirtying any internal
> + * state. When read() is done on this temporary end of stream the kernel
> driver
> + * should return ENOMSG from read(). Userspace can wait for more data
> (which may
> + * never come) by using poll.
> + *
> + * Once in STOP_COPY the migration data FD has a permanent end of
> stream
> + * signaled in the usual way by read() always returning 0 and poll always
> + * returning readable. ENOMSG may not be returned in STOP_COPY.
> Support
> + * for this ioctl is optional.

Isn't mandatory if the driver claims support for VFIO_MIGRATION_PRE_COPY?

Other than that looks fine to me.

Reviewed-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>