Re: [PATCH 3/6] KVM: arm64: Make stage2_pte_cacheable() return false when S2_AS_S1 is set

Marc Zyngier
2026-01-23T13:21:26Z

On Thu, 22 Jan 2026 15:04:17 +0000,
Will Deacon <will@kernel.org> wrote:
>
> On Mon, Jan 19, 2026 at 10:56:48AM +0000, Marc Zyngier wrote:
> > We cannot tell from the S2 attributes whether what we map is memory
> > or not when S2_AS_S1 is set, and issuing a CMO on device memory may
> > not be the best idea.
> >
> > In this particular case, pretend that the mapping isn't cacheable.
> >
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> > arch/arm64/kvm/hyp/pgtable.c | 8 ++++++++
> > 1 file changed, 8 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 3c5f399b1b986..07561a227f75e 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -911,6 +911,14 @@ static void stage2_unmap_put_pte(const struct kvm_pgtable_visit_ctx *ctx,
> > static bool stage2_pte_cacheable(struct kvm_pgtable *pgt, kvm_pte_t pte)
> > {
> > u64 memattr = pte & KVM_PTE_LEAF_ATTR_LO_S2_MEMATTR;
> > +
> > + /*
> > + * With S2_AS_S1, we have no idea whether the OA is actual memory or
> > + * a device. Don't even try a CMO on that.
> > + */
> > + if (pgt->flags & KVM_PGTABLE_S2_AS_S1)
> > + return false;
> > +
> > return kvm_pte_valid(pte) && memattr == KVM_S2_MEMATTR(pgt, NORMAL);
>
> Since KVM_PGTABLE_S2_AS_S1 is only set for the pKVM host stage-2 and
> that doesn't provide any cache maintenance callbacks in its
> kvm_pgtable_mm_ops, I don't think the current code is actually broken,

Frankly, I have no idea how I came to such a stupid conclusion. My
only way out is to blame Metallica (don't listen to that shit, kids!).

> although it's harmless to add the check (and the comment might benefit
> from some additional rewording?).

I don't think there is much value in adding something that is not
useful. I'll drop that patch altogether.

Thanks,

M.

--
Without deviation from the norm, progress is not possible.