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

[Alex Williamson <alex.williamson@redhat.com>]

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,

Alex