Re: [PATCH v9 3/6] KVM: arm64: Block cacheable PFNMAP mapping

Will Deacon
2025-07-04T16:24:24Z

On Fri, Jul 04, 2025 at 02:21:32PM +0200, David Hildenbrand wrote:
> On 30.06.25 14:25, Jason Gunthorpe wrote:
> > On Mon, Jun 30, 2025 at 01:56:43AM +0000, Ankit Agrawal wrote:
> > > > Sorry for the drive-by comment, but I was looking at this old series from
> > > > Paolo (look at the cover letter and patch 5):
> > > >
> > > > https://lore.kernel.org/r/20250109133817.314401-1-pbonzini@redhat.com
> > > >
> > > > in which he points out that the arm64 get_vma_page_shift() function
> > > > incorrectly assumes that a VM_PFNMAP VMA is physically contiguous, which
> > > > may not be the case if a driver calls remap_pfn_range() to mess around
> > > > with mappings within the VMA. I think that implies that the optimisation
> > > > in 2aa53d68cee6 ("KVM: arm64: Try stage2 block mapping for host device
> > > > MMIO") is unsound.
> > >
> > > Hm yeah, that does seem problematic. Perhaps we need a new
> > > vma flag that could help the driver communicate to the KVM that the
> > > mapping is contiguous and it can go ahead with the optimization?
> > > E.g. something similar to VM_ALLOW_ANY_UNCACHED.
> >
> > I think Paolo has the right direction - remove any attempts by KVM to
> > expand contiguity, it should only copy the primary's PTEs and rely on
> > the primary to discover contiguity. No new flags.
>
> 100%

The part I don't understand, however, is that I can't see an MMU notifier
anywhere on the successful remap_pfn_range() path. So if a driver is
using that interface to change the mapping properties of a VM_PFNMAP VMA,
how do we ensure that the guest doesn't use whatever stale mappings it's
faulted in previously? Did I just miss something?

Will