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Date:   Fri, 9 Jun 2023 14:51:09 +0000
From:   Oliver Upton <oliver.upton@linux.dev>
To:     Chun-Tse Shao <ctshao@google.com>
Cc:     linux-kernel@vger.kernel.org, yuzhao@google.com,
        Marc Zyngier <maz@kernel.org>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Zenghui Yu <yuzenghui@huawei.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Ben Gardon <bgardon@google.com>,
        Gavin Shan <gshan@redhat.com>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev
Subject: Re: [PATCH v1 3/3] KVM: arm64: Using rcu_read_lock() for
 kvm_pgtable_stage2_mkyoung()
Message-ID: <ZIM8XccN31XSb+Qh@linux.dev>
References: <20230608220558.39094-1-ctshao@google.com>
 <20230608220558.39094-4-ctshao@google.com>
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On Thu, Jun 08, 2023 at 03:05:41PM -0700, Chun-Tse Shao wrote:
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 3b9d4d24c361..0f7ea66fb894 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1437,10 +1437,10 @@ static void handle_access_fault(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa)
>  
>  	trace_kvm_access_fault(fault_ipa);
>  
> -	read_lock(&vcpu->kvm->mmu_lock);
> +	rcu_read_lock();
>  	mmu = vcpu->arch.hw_mmu;
>  	pte = kvm_pgtable_stage2_mkyoung(mmu->pgt, fault_ipa);
> -	read_unlock(&vcpu->kvm->mmu_lock);
> +	rcu_read_unlock();

What is the point of acquiring the RCU read lock here?
kvm_pgtable_walk_{begin,end}() already do the exact same for any
'shared' walk.

I agree with Marc that this warrants some very clear benchmark data
showing the value of the change. As I had mentioned to Yu, I already
implemented this for my own purposes, but wasn't able to see a
significant improvement over acquiring the MMU lock for read.

-- 
Thanks,
Oliver