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

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Message-ID: <Y4Zw/J3srTsZ57P7@google.com>
Date:   Tue, 29 Nov 2022 12:52:12 -0800
From:   Ricardo Koller <ricarkol@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] KVM: arm64: Don't serialize if the access flag isn't
 set

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@...ux.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
> 
>