Re: [Qemu-devel] [PATCH v2 0/4] Balloon inhibit enhancements, vfio restriction

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Alex Williamson (alex.williamson@redhat.com) wrote:
> On Tue, 31 Jul 2018 15:29:17 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Mon, Jul 30, 2018 at 05:13:26PM -0600, Alex Williamson wrote:
> > > v2:
> > >  - Use atomic ops for balloon inhibit counter (Peter)
> > >  - Allow endpoint driver opt-in for ballooning, vfio-ccw opt-in by
> > >    default, vfio-pci opt-in by device option, only allowed for mdev
> > >    devices, no support added for platform as there are no platform
> > >    mdev devices.
> > > 
> > > See patch 3/4 for detailed explanation why ballooning and device
> > > assignment typically don't mix.  If this eventually changes, flags
> > > on the iommu info struct or perhaps device info struct can inform
> > > us for automatic opt-in.  Thanks,
> > > 
> > > Alex  
> > 
> > So this patch seems to block ballooning when vfio is added.
> > But what if balloon is added and inflated first?
> 
> Good point.
>  
> > I'd suggest making qemu_balloon_inhibit fail in that case,
> > and then vfio realize will fail as well.
> 
> That might be the correct behavior for vfio, but I wonder about the
> existing postcopy use case.  Dave Gilbert, what do you think?  We might
> need a separate interface for callers that cannot tolerate existing
> ballooned pages.  Of course we'll also need another atomic counter to
> keep a tally of ballooned pages.  Thanks,

For postcopy, preinflation isn't a problem; our only issue is ballooning
during the postcopy phase itself.

Dave

> Alex
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK