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

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

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.

> ---
> 
> Alex Williamson (3):
>       balloon: Allow nested inhibits
>       kvm: Use inhibit to prevent ballooning without synchronous mmu
>       vfio: Inhibit ballooning
> 
> 
>  accel/kvm/kvm-all.c        |    4 ++++
>  balloon.c                  |    7 ++++---
>  hw/vfio/common.c           |    6 ++++++
>  hw/virtio/virtio-balloon.c |    4 +---
>  4 files changed, 15 insertions(+), 6 deletions(-)