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

[Alex Williamson]

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,

Alex