Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 09 Jun 2023 08:44:07 +0100
Message-ID: <878rctnkqg.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Chun-Tse Shao <ctshao@google.com>
Cc:     linux-kernel@vger.kernel.org, yuzhao@google.com,
        oliver.upton@linux.dev, 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()
In-Reply-To: <20230608220558.39094-4-ctshao@google.com>
References: <20230608220558.39094-1-ctshao@google.com>
        <20230608220558.39094-4-ctshao@google.com>
User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue)
 FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/28.2
 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO)
MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue")
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

On Thu, 08 Jun 2023 23:05:41 +0100,
Chun-Tse Shao <ctshao@google.com> wrote:
> 
> Access bit is RCU safe and can be set without taking kvm->mmu_lock().

Please explain why. What happens when the page tables are *not* RCU
controlled, such as in the pKVM case?

> Replacing existing kvm->mmu_lock() with rcu_read_lock() for better
> performance.

Please define "better performance", quote workloads, figures, HW setup
and point to a reproducer. Please add a cover letter to your patch
series explaining the context this happens in.

Also, I'm getting increasingly annoyed by the lack of coordination
between seamingly overlapping patch series (this, Yu's, Anish's and
Vipin's), all from a single company.

Surely you can talk to each other and devise a coordinated approach?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.