On Wed, 18 Jul 2018 14:48:03 +0800 Peter Xu <peterx@xxxxxxxxxx> 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, > > IMHO patches 1-2 are good cleanup as standalone patches... > > I totally have no idea on whether people would like to use vfio-pci > and the balloon device at the same time. After all vfio-pci are > majorly for performance players, then I would vaguely guess that they > don't really care thin provisioning of memory at all, hence the usage > scenario might not exist much. Is that the major reason that we'd > just like to disable it (which makes sense to me)? Don't people use vfio-pci as well if they want some special capabilities from the passed-through device? (At least that's the main use case for vfio-ccw, not any performance considerations.) > > I'm wondering what if want to do that somehow some day... Whether > it'll work if we just let vfio-pci devices to register some guest > memory invalidation hook (just like the iommu notifiers, but for guest > memory address space instead), then we map/unmap the IOMMU pages there > for vfio-pci device to make sure the inflated balloon pages are not > mapped and also make sure new pages are remapped with correct HPA > after deflated. This is a pure question out of my curiosity, and for > sure it makes little sense if the answer of the first question above > is positive. > > Thanks, >
