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

[David Hildenbrand]

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?

-- 

Thanks,

David / dhildenb