Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 07, 2018 at 01:53:03PM -0600, Alex Williamson wrote:
> On Tue, 7 Aug 2018 22:44:56 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Tue, Aug 07, 2018 at 01:31:21PM -0600, Alex Williamson wrote:
> > > v3:
> > >  - Drop "nested" term in commit log (David)
> > >  - Adopt suggested wording in ccw code (Cornelia)
> > >  - Explain balloon inhibitor usage in vfio common (Peter)
> > >  - Fix to call inhibitor prior to re-using existing containers
> > >    to avoid gap that pinning may have occurred in set container
> > >    ioctl (self) - Peter, this change is the reason I didn't
> > >    include your R-b.
> > >  - Add R-b to patches 1 & 2
> > > 
> > > 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  
> > 
> > One of the issues with pass-through is that it breaks overcommit
> > through swap. ballooning seems to offer one solution, instead of
> > making it work this patch just attempts to block ballooning.
> > 
> > I guess it's better than corrupting memory but I personally find this
> > approach disappointing.
> 
> Memory hotplug is the way to achieve variable density with assigned
> device VMs, otherwise look towards approaches like mdev and shared
> virtual addresses with PASID support.  We cannot shoehorn page faulting
> without both hardware and software support.  Some class of "legacy"
> device assignment will always have this incompatibility.  Thanks,
> 
> Alex

I'm not sure I agree.

At least with VTD, it seems entirely possible to change e.g. a PMD
atomically to point to a different set of PTEs, then flush.
That will allow removing memory at high granularity for
an arbitrary device without mdev or PASID dependency.

I suspect most IOMMUs are like this.

IIUC doing that within guest right now will cause a range to be unmapped
and them mapped again, which I suspect only works if we are lucky and
device does not access the range during this time.

So at some level it's a theoretical bug we would do well to fix,
and then we can support ballooning better.

-- 
MST