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
