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
