Re: [PATCH 4/9] vfio/migration: Skip pre-copy if dirty page tracking is not supported

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, May 17, 2022 at 10:00:45AM -0600, Alex Williamson wrote:

> > This is really intended to be a NOP from where things are now, as if
> > you use mlx5 live migration without a patch like this then it causes a
> > botched pre-copy since everything just ends up permanently dirty.
> > 
> > If it makes more sense we could abort the pre-copy too - in the end
> > there will be dirty tracking so I don't know if I'd invest in a big
> > adventure to fully define non-dirty tracking migration.
> 
> How is pre-copy currently "botched" without a patch like this?  If it's
> simply that the pre-copy doesn't converge and the downtime constraints
> don't allow the VM to enter stop-and-copy, that's the expected behavior
> AIUI, and supports backwards compatibility with existing SLAs.

It means it always fails - that certainly isn't working live
migration. There is no point in trying to converge something that we
already know will never converge.

> I'm assuming that by setting this new skip_precopy flag that we're
> forcing the VM to move to stop-and-copy, regardless of any other SLA
> constraints placed on the migration.  

That does seem like a defect in this patch, any SLA constraints should
still all be checked under the assumption all ram is dirty.

> It seems like a better solution would be to expose to management
> tools that the VM contains a device that does not support the
> pre-copy phase so that downtime expectations can be adjusted.

I don't expect this to be a real use case though..

Remember, you asked for this patch when you wanted qemu to have good
behavior when kernel support for legacy dirty tracking is removed
before we merge v2 support.

Thanks,
Jason