Re: [PATCH V1 vfio 02/14] vfio: Extend the device migration protocol with PRE_COPY

[Yishai Hadas <yishaih@nvidia.com>]

On 01/12/2022 2:51, Jason Gunthorpe wrote:
> On Wed, Nov 30, 2022 at 03:22:40PM -0700, Alex Williamson wrote:
>> On Thu, 24 Nov 2022 19:39:20 +0200
>> Yishai Hadas <yishaih@nvidia.com> wrote:
>>> +/**
>>> + * 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 mandatory 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 a must to leave PRE_COPY for STOP_COPY only after this field reach
>>> + * zero.
>>
>> Is this actually a requirement that's compatible with current QEMU
>> behavior?  It's my impression that a user can force the migration to
>> move to STOP_COPY at any point in time.  Thanks,
> I think it is a typo
>
> It should be explaining that leaving PRE_COPY early will make things
> slower, but is not a functional problem.
>
> Jason

Right

I''ll rephrase as part of V2 as of below.

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.

Yishai