Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 09, 2018 at 05:37:58PM +0800, Peter Xu wrote:
> On Thu, Aug 09, 2018 at 12:23:43PM +0300, Michael S. Tsirkin wrote:
> > On Wed, Aug 08, 2018 at 11:45:43AM +0800, Peter Xu wrote:
> > > On Wed, Aug 08, 2018 at 12:58:32AM +0300, Michael S. Tsirkin wrote:
> > > > 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.
> > > 
> > > My understanding is that the guest driver should prohibit this kind of
> > > operation (say, modifying PMD).
> > 
> > Interesting.  Which part of the VTD spec prohibits this?
> > 
> > > Actually I don't see how it can
> > > happen in Linux if the kernel drivers always call the IOMMU API since
> > > there are only map/unmap APIs rather than this atomic-modify API.
> > 
> > It could happen with a non-Linux guest which might have a different API.
> > 
> > > The thing is that IMHO it's the guest driver's responsibility to make
> > > sure the pages will never be used by the device before it removes the
> > > entry (including modifying the PMD since that actually removes all the
> > > entries on the old PMD).
> > 
> > If you switch PMDs atomically from one set of valid PTEs to another,
> > then flush, then as far as I could see it just works in the hardware
> > VTD, but not in the emulated VTD. So that's a difference in
> > behaviour. Maybe we are lucky and no one does that.
> 
> Yes, but AFAICT that's also the best we can have now since the
> userspace QEMU (or say, the VT-d emulation code) cannot really modify
> a real PMD that the hardware uses - it can only call the VFIO APIs,
> and finally it boils down again to the host kernel IOMMU APIs to do
> map or unmap only.  So it's a impossible task until we provide such an
> interface through the whole IOMMU/VFIO/... stack just like what you
> have discussed in the other thread.
> 
> Thanks,


This would need host kernel support, yes.

> -- 
> Peter Xu