Re: [PATCH v2] KVM: arm64: Optimize protected mode and FWB

[Fuad Tabba]

On Tue, 21 Jul 2026 at 11:19, Mostafa Saleh <smostafa@xxxxxxxxxx> wrote:
>
> On Tue, Jul 21, 2026 at 11:10:07AM +0100, Marc Zyngier wrote:
> > On Tue, 21 Jul 2026 09:56:45 +0100,
> > Mostafa Saleh <smostafa@xxxxxxxxxx> wrote:
> > >
> > > KVM opportunistically enables FWB if supported by the system for guest
> > > VMs, which allows it to elude cache maintenance for data as they are
> > > forced to be cacheable from stage-2.
> > > In that case, __clean_dcache_guest_page() will immediately return.
> > > However in protected mode, before calling __clean_dcache_guest_page()
> > > it loops over the range and fix_map/unmap it, issuing TLB
> > > invalidations, dsb() and isb() unnecessarily.
> > >
> > > This can be optimized by returning early if FWB is supported,
> > > kvm_pgtable_stage2_map() already issues dsb() and tlb invalidation
> > > functions issue dsb() for the unmap path.
> > >
> > > Signed-off-by: Mostafa Saleh <smostafa@xxxxxxxxxx>
> > > Reviewed-by: Fuad Tabba <fuad.tabba@xxxxxxxxx>
> > > ---
> > >  arch/arm64/kvm/hyp/nvhe/mem_protect.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > >
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > > index 4e329e39a695..049a09be2bbf 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > > @@ -261,6 +261,13 @@ static void __apply_guest_page(void *va, size_t size,
> > >
> > >  static void clean_dcache_guest_page(void *va, size_t size)
> > >  {
> > > +   /*
> > > +    * Guest stage-2 uses FWB if available, making it safe to elide CMOs.
> > > +    * In contrast, the host stage-2 never has FWB enabled.
> >
> > Beeeeeeep!!! Since a373930ec9406 ("KVM: arm64: Switch pKVM host S2
> > over to KVM_PGTABLE_S2_AS_S1"), FWB is enabled everywhere when
> > available.
> >
> > Not sure that changes anything, but the comment is factually wrong.
>
> Ah, I see it uses KVM_PGTABLE_S2_AS_S1 in that case, that should not
> change the patch, I will need to update the comment.

Sorry, this confusing is partially my fault. The original comment in v1

>  /* See __clean_dcache_guest_page() */

wasn't very clear to me, since I wasn't sure what I was looking for in
__clean_dcache_guest_page(). Talking to Mostafa in person, he's
referring to the comment _inside_ __clean_dcache_guest_page. I think
the best fix is to rephase as

+  /* See comment in __clean_dcache_guest_page() */

This would save duplication and having to repeat the comment.

Cheers,
/fuad

> Thanks,
> Mostafa
>
> >
> > Thanks,
> >
> >       M.
> >
> > --
> > Without deviation from the norm, progress is not possible.