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

[Fuad Tabba]

On Mon, 20 Jul 2026 at 21:35, 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>
> ---
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index 4e329e39a695..6e9229106a25 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -261,6 +261,10 @@ static void __apply_guest_page(void *va, size_t size,
>
>  static void clean_dcache_guest_page(void *va, size_t size)
>  {
> +       /* See __clean_dcache_guest_page() */

This looks good to me. One comment: this file also has
hyp_poison_page(), which deliberately avoids
__clean_dcache_guest_page() because the FWB elision would be wrong
there (it even notes "Prefer kvm_flush_dcache_to_poc() over
__clean_dcache_guest_page()"). Given the two opposite treatments in
the same file, could this comment say why eliding is safe here, e.g.
that the consumer is the guest via the FWB-forced stage-2? It would
save the next reader (if they're anything like me) from reconciling
the two.

That fuller comment could also replace "See
__clean_dcache_guest_page()", which mostly just points back at the
function passed in the __apply_guest_page() call below.

With that fixed:

Reviewed-by: Fuad Tabba <fuad.tabba@xxxxxxxxx>

Cheers,
/fuad

> +       if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
> +               return;
> +
>         __apply_guest_page(va, size, __clean_dcache_guest_page);
>  }
>
> --
> 2.55.0.229.g6434b31f56-goog
>