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

[Michael S. Tsirkin]

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.

>  If not, I would see it a guest kernel bug
> instead of the bug in the emulation code.
> 
> Thanks,
> 
> -- 
> Peter Xu