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

Tian, Kevin
2022-12-02T08:48:59Z

> From: Yishai Hadas <yishaih@nvidia.com>
> Sent: Thursday, December 1, 2022 11:29 PM
>
> +/**
> + * 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.

'slower' because partially transferred initial state is wasted and a full
state transfer is still required in STOP_COPY?

> + *
> + * 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.

I didn't get what the last sentence is trying to say. By definition those
fields have nothing to do with the transferred data in STOP_COPY.

is there an example what a silly driver might do w/o this caveat?

Except above this looks good to me:

Reviewed-by: Kevin Tian <kevin.tian@intel.com>