linux-kernel - Re: [PATCH v1 3/3] KVM: arm64: Using rcu_read_lock() for kvm_pgtable_stage2

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Message-ID: <ZIM8XccN31XSb+Qh@linux.dev>
Date:   Fri, 9 Jun 2023 14:51:09 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Chun-Tse Shao <ctshao@...gle.com>
Cc:     linux-kernel@...r.kernel.org, yuzhao@...gle.com,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Ben Gardon <bgardon@...gle.com>,
        Gavin Shan <gshan@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v1 3/3] KVM: arm64: Using rcu_read_lock() for
 kvm_pgtable_stage2_mkyoung()

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