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

[Jason Gunthorpe <jgg@nvidia.com>]

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