Re: [RFC 7/7] migration: call qemu_savevm_state_pending_exact() with the guest stopped

[Jason Gunthorpe via <qemu-devel@nongnu.org>]

On Fri, Oct 14, 2022 at 01:29:51PM +0100, Joao Martins wrote:
> On 14/10/2022 12:28, Juan Quintela wrote:
> > Joao Martins <joao.m.martins@oracle.com> wrote:
> >> On 13/10/2022 17:08, Juan Quintela wrote:
> >>> Oops.  My understanding was that once the guest is stopped you can say
> >>> how big is it. 
> > 
> > Hi
> > 
> >> It's worth keeping in mind that conceptually a VF won't stop (e.g. DMA) until
> >> really told through VFIO. So, stopping CPUs (guest) just means that guest code
> >> does not arm the VF for more I/O but still is a weak guarantee as VF still has
> >> outstanding I/O to deal with until VFIO tells it to quiesce DMA (for devices
> >> that support it).
> > 
> > How can we make sure about that?
> > 
> > i.e. I know I have a vfio device.  I need two things:
> > - in the iterative stage, I eed to check the size, but a estimate is ok.
> >   for example with RAM, we use whatever is the size of the dirty bitmap
> >   at that moment.
> >   If the estimated size is smaller than the theshold, we
> >    * stop the guest
> >    * sync dirty bitmap
> >    * now we test the (real) dirty bitmap size
> > 
> > How can we do something like that with a vfio device.
> > 
> You would have an extra intermediate step that stops the VF prior to asking
> the device state length. What I am not sure is whether stopping
> vCPUs can be skipped as an optimization.

It cannot, if you want to stop the VFIO device you must also stop the
vCPUs because the device is not required to respond properly to MMIO
operations when stopped.

> > My understanding from NVidia folks was that newer firmware have an ioctl
> > to return than information.
> 
> Maybe there's something new. I was thinking from this here:

Juan is talking about the ioctl we had in the pre-copy series.

I expect it to come into some different form to support this RFC.

Jason