Re: [PATCH v9 13/17] KVM: arm64: Handle guest_memfd()-backed guest page faults

[Fuad Tabba <tabba@google.com>]

Hi David,

On Wed, 21 May 2025 at 09:04, David Hildenbrand <david@redhat.com> wrote:
>
> On 13.05.25 18:34, Fuad Tabba wrote:
> > Add arm64 support for handling guest page faults on guest_memfd
> > backed memslots.
> >
> > For now, the fault granule is restricted to PAGE_SIZE.
> >
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
>
> [...]
>
> > +     if (!is_gmem) {
>
> Should we add a comment somewhere, stating that we don't support VMs
> with private memory, so if we have a gmem, all faults are routed through
> that?

I guess this is related to the other thread we had. This would handle
private memory correctly. It's just that for arm64 as it is, having
private memory isn't that useful.

There might be a use-case where a user would create a
guest_memfd-backed slot that supports private memory, and one that
doesn't, which only the guest would use. I doubt that that's actually
useful, but it would work and behave as expected.

Cheers,
/fuad

> > +             mmap_read_lock(current->mm);
> > +             vma = vma_lookup(current->mm, hva);
> > +             if (unlikely(!vma)) {
> > +                     kvm_err("Failed to find VMA for hva 0x%lx\n", hva);
> > +                     mmap_read_unlock(current->mm);
> > +                     return -EFAULT;
> > +             }
> > +
> > +             vfio_allow_any_uc = vma->vm_flags & VM_ALLOW_ANY_UNCACHED;
> > +             mte_allowed = kvm_vma_mte_allowed(vma);
>
> --
> Cheers,
>
> David / dhildenb
>