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(-)
