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

[Mostafa Saleh <smostafa@google.com>]

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@google.com> 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@google.com>
> > Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
> > ---
> >  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.

Thanks,
Mostafa

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