Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements

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

On Mon, Jul 30, 2018 at 03:54:04PM +0200, David Hildenbrand wrote:
> On 30.07.2018 15:34, Michael S. Tsirkin wrote:
> > On Tue, Jul 17, 2018 at 04:47:31PM -0600, Alex Williamson wrote:
> >> Directly assigned vfio devices have never been compatible with
> >> ballooning.  Zapping MADV_DONTNEED pages happens completely
> >> independent of vfio page pinning and IOMMU mapping, leaving us with
> >> inconsistent GPA to HPA mapping between vCPUs and assigned devices
> >> when the balloon deflates.  Mediated devices can theoretically do
> >> better, if we make the assumption that the mdev vendor driver is fully
> >> synchronized to the actual working set of the guest driver.  In that
> >> case the guest balloon driver should never be able to allocate an mdev
> >> pinned page for balloon inflation.  Unfortunately, QEMU can't know the
> >> workings of the vendor driver pinning, and doesn't actually know the
> >> difference between mdev devices and directly assigned devices.  Until
> >> we can sort out how the vfio IOMMU backend can tell us if ballooning
> >> is safe, the best approach is to disabling ballooning any time a vfio
> >> devices is attached.
> >>
> >> To do that, simply make the balloon inhibitor a counter rather than a
> >> boolean, fixup a case where KVM can then simply use the inhibit
> >> interface, and inhibit ballooning any time a vfio device is attached.
> >> I'm expecting we'll expose some sort of flag similar to
> >> KVM_CAP_SYNC_MMU from the vfio IOMMU for cases where we can resolve
> >> this.  An addition we could consider here would be yet another device
> >> option for vfio, such as x-disable-balloon-inhibit, in case there are
> >> mdev devices that behave in a manner compatible with ballooning.
> >>
> >> Please let me know if this looks like a good idea.  Thanks,
> >>
> >> Alex
> > 
> > It's probably the only a reasonable thing to do for this release.
> > 
> > Long term however, why can't balloon notify vfio as pages are
> > added and removed? VFIO could update its mappings then.
> 
> What if the guest is rebooted and pages are silently getting reused
> without getting a deflation request first?

Good point. To handle we'd need to deflate fully on
on device reset, allowing access to all memory again.

> -- 
> 
> Thanks,
> 
> David / dhildenb