Re: [PATCH Kernel v22 0/8] Add UAPIs to support migration for VFIO devices

[Kirti Wankhede]

On 5/29/2020 4:29 AM, Alex Williamson wrote:
> On Wed, 27 May 2020 09:48:22 +0100
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > * Yan Zhao (yan.y.zhao@intel.com) wrote:
> > > BTW, for viommu, the downtime data is as below. under the same network
> > > condition and guest memory size, and no running dirty data/memory produced
> > > by device.
> > > (1) viommu off
> > > single-round dirty query: downtime ~100ms
> >
> > Fine.
> >
> > > (2) viommu on
> > > single-round dirty query: downtime 58s
> >
> > Youch.
>
> Double Youch!  But we believe this is because we're getting the dirty
> bitmap one IOMMU leaf page at a time, right?  We've enable the kernel
> to get a dirty bitmap across multiple mappings, but QEMU isn't yet
> taking advantage of it.  Do I have this correct?  Thanks,

That's correct.

Thanks,
Kirti