Re: [PATCH 2/4] KVM: arm64: Don't serialize if the access flag isn't set

Ricardo Koller
2022-11-29T20:52:16Z

On Tue, Nov 29, 2022 at 07:19:44PM +0000, Oliver Upton wrote:
> Of course, if the PTE wasn't changed then there are absolutely no
> serialization requirements. Skip the DSB for an unsuccessful update to
> the access flag.
>
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
> arch/arm64/kvm/hyp/pgtable.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index b11cf2c618a6..9626f615d9b8 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -1094,9 +1094,13 @@ int kvm_pgtable_stage2_wrprotect(struct kvm_pgtable *pgt, u64 addr, u64 size)
> kvm_pte_t kvm_pgtable_stage2_mkyoung(struct kvm_pgtable *pgt, u64 addr)
> {
> kvm_pte_t pte = 0;
> - stage2_update_leaf_attrs(pgt, addr, 1, KVM_PTE_LEAF_ATTR_LO_S2_AF, 0,
> - &pte, NULL, 0);
> - dsb(ishst);
> + int ret;
> +
> + ret = stage2_update_leaf_attrs(pgt, addr, 1, KVM_PTE_LEAF_ATTR_LO_S2_AF, 0,
> + &pte, NULL, 0);
> + if (!ret)
> + dsb(ishst);

At the moment, the only reason for stage2_update_leaf_attrs() to not
update the PTE is if it's not valid:

if (!kvm_pte_valid(pte))
return 0;

I guess you could check that as well:

+ if (!ret || kvm_pte_valid(pte))
+ dsb(ishst);

> +
> return pte;
> }
>
> --
> 2.38.1.584.g0f3c55d4c2-goog
>
>