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

From: Cornelia Huck
Subject: Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements
Date: Wed, 18 Jul 2018 11:36:40 +0200

On Wed, 18 Jul 2018 14:48:03 +0800
Peter Xu <address@hidden> 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,

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