linux-kernel - Re: [PATCH v2 2/2] KVM: arm/arm64: Skip updating PTE entry if no change

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Message-ID: <3ef331d7-d31a-a61b-7ad0-eb7c8bee10bd@arm.com>
Date:   Mon, 13 Aug 2018 10:47:15 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Punit Agrawal <punit.agrawal@....com>, kvmarm@...ts.cs.columbia.edu
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        marc.zyngier@....com, christoffer.dall@....com,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 2/2] KVM: arm/arm64: Skip updating PTE entry if no
 change

On 08/13/2018 10:40 AM, Punit Agrawal wrote:
> When there is contention on faulting in a particular page table entry
> at stage 2, the break-before-make requirement of the architecture can
> lead to additional refaulting due to TLB invalidation.
> 
> Avoid this by skipping a page table update if the new value of the PTE
> matches the previous value.
> 
> Fixes: d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
> Change-Id: I28e17daf394a4821b13c2cf8726bf72bf30434f9
> Signed-off-by: Punit Agrawal <punit.agrawal@....com>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Christoffer Dall <christoffer.dall@....com>
> Cc: Suzuki Poulose <suzuki.poulose@....com>
> Cc: stable@...r.kernel.org
> ---
>   virt/kvm/arm/mmu.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 2ab977edc63c..d0a9dccc3793 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1120,6 +1120,10 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>   	/* Create 2nd stage page table mapping - Level 3 */
>   	old_pte = *pte;
>   	if (pte_present(old_pte)) {
> +		/* Skip page table update if there is no change */
> +		if (pte_val(old_pte) == pte_val(*new_pte))
> +			goto out;
> +
>   		kvm_set_pte(pte, __pte(0));
>   		kvm_tlb_flush_vmid_ipa(kvm, addr);
>   	} else {
> @@ -1127,6 +1131,7 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>   	}
>   
>   	kvm_set_pte(pte, *new_pte);
> +out:

Similar comments as the previous patch, either way:

Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>