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Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements |
Date: |
Mon, 30 Jul 2018 16:34:09 +0300 |
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(-)
Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements,
Michael S. Tsirkin <=
Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements, Alex Williamson, 2018/07/30