All of lore.kernel.org
 help / color / mirror / Atom feed
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()
Date: Fri, 9 Jun 2023 14:51:09 +0000	[thread overview]
Message-ID: <ZIM8XccN31XSb+Qh@linux.dev> (raw)
In-Reply-To: <20230608220558.39094-4-ctshao@google.com>

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

WARNING: multiple messages have this Message-ID (diff)
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()
Date: Fri, 9 Jun 2023 14:51:09 +0000	[thread overview]
Message-ID: <ZIM8XccN31XSb+Qh@linux.dev> (raw)
In-Reply-To: <20230608220558.39094-4-ctshao@google.com>

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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  parent reply	other threads:[~2023-06-09 14:51 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-06-08 22:05 [PATCH v1 1/3] KVM: arm64: Consistently use free_removed_table() for stage-2 Chun-Tse Shao
2023-06-08 22:05 ` Chun-Tse Shao
2023-06-08 22:05 ` [PATCH v1 2/3] KVM: arm64: Only initiate walk if page_count() > 1 in free_removed_table() Chun-Tse Shao
2023-06-08 22:05   ` Chun-Tse Shao
2023-06-08 22:05 ` [PATCH v1 3/3] KVM: arm64: Using rcu_read_lock() for kvm_pgtable_stage2_mkyoung() Chun-Tse Shao
2023-06-08 22:05   ` Chun-Tse Shao
2023-06-09  7:44   ` Marc Zyngier
2023-06-09  7:44     ` Marc Zyngier
2023-06-09 22:58     ` Chun-Tse Shao
2023-06-09 22:58       ` Chun-Tse Shao
2023-06-09 14:51   ` Oliver Upton [this message]
2023-06-09 14:51     ` Oliver Upton
2023-06-08 22:13 ` [PATCH v1 1/3] KVM: arm64: Consistently use free_removed_table() for stage-2 Yu Zhao
2023-06-08 22:13   ` Yu Zhao

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=ZIM8XccN31XSb+Qh@linux.dev \
    --to=oliver.upton@linux.dev \
    --cc=bgardon@google.com \
    --cc=catalin.marinas@arm.com \
    --cc=ctshao@google.com \
    --cc=gshan@redhat.com \
    --cc=james.morse@arm.com \
    --cc=kvmarm@lists.linux.dev \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=maz@kernel.org \
    --cc=suzuki.poulose@arm.com \
    --cc=will@kernel.org \
    --cc=yuzenghui@huawei.com \
    --cc=yuzhao@google.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.