[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restr
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction |
Date: |
Wed, 8 Aug 2018 00:58:32 +0300 |
On Tue, Aug 07, 2018 at 01:53:03PM -0600, Alex Williamson wrote:
> On Tue, 7 Aug 2018 22:44:56 +0300
> "Michael S. Tsirkin" <address@hidden> wrote:
>
> > On Tue, Aug 07, 2018 at 01:31:21PM -0600, Alex Williamson wrote:
> > > v3:
> > > - Drop "nested" term in commit log (David)
> > > - Adopt suggested wording in ccw code (Cornelia)
> > > - Explain balloon inhibitor usage in vfio common (Peter)
> > > - Fix to call inhibitor prior to re-using existing containers
> > > to avoid gap that pinning may have occurred in set container
> > > ioctl (self) - Peter, this change is the reason I didn't
> > > include your R-b.
> > > - Add R-b to patches 1 & 2
> > >
> > > v2:
> > > - Use atomic ops for balloon inhibit counter (Peter)
> > > - Allow endpoint driver opt-in for ballooning, vfio-ccw opt-in by
> > > default, vfio-pci opt-in by device option, only allowed for mdev
> > > devices, no support added for platform as there are no platform
> > > mdev devices.
> > >
> > > See patch 3/4 for detailed explanation why ballooning and device
> > > assignment typically don't mix. If this eventually changes, flags
> > > on the iommu info struct or perhaps device info struct can inform
> > > us for automatic opt-in. Thanks,
> > >
> > > Alex
> >
> > One of the issues with pass-through is that it breaks overcommit
> > through swap. ballooning seems to offer one solution, instead of
> > making it work this patch just attempts to block ballooning.
> >
> > I guess it's better than corrupting memory but I personally find this
> > approach disappointing.
>
> Memory hotplug is the way to achieve variable density with assigned
> device VMs, otherwise look towards approaches like mdev and shared
> virtual addresses with PASID support. We cannot shoehorn page faulting
> without both hardware and software support. Some class of "legacy"
> device assignment will always have this incompatibility. Thanks,
>
> Alex
I'm not sure I agree.
At least with VTD, it seems entirely possible to change e.g. a PMD
atomically to point to a different set of PTEs, then flush.
That will allow removing memory at high granularity for
an arbitrary device without mdev or PASID dependency.
I suspect most IOMMUs are like this.
IIUC doing that within guest right now will cause a range to be unmapped
and them mapped again, which I suspect only works if we are lucky and
device does not access the range during this time.
So at some level it's a theoretical bug we would do well to fix,
and then we can support ballooning better.
--
MST
- Re: [Qemu-devel] [PATCH v3 1/4] balloon: Allow multiple inhibit users, (continued)
[Qemu-devel] [PATCH v3 2/4] kvm: Use inhibit to prevent ballooning without synchronous mmu, Alex Williamson, 2018/08/07
[Qemu-devel] [PATCH v3 3/4] vfio: Inhibit ballooning based on group attachment to a container, Alex Williamson, 2018/08/07
[Qemu-devel] [PATCH v3 4/4] vfio/ccw/pci: Allow devices to opt-in for ballooning, Alex Williamson, 2018/08/07
Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction, Michael S. Tsirkin, 2018/08/07
- Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction, Alex Williamson, 2018/08/07
- Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction, Peter Xu, 2018/08/07
- Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction, Alex Williamson, 2018/08/08
- Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction, Michael S. Tsirkin, 2018/08/09
Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction, Michael S. Tsirkin, 2018/08/09
Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction, Peter Xu, 2018/08/09
Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction, Michael S. Tsirkin, 2018/08/09