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

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Message-ID: <CAJpZYjVcDAU94Abx7BUV-nZeTJ88ggYs=n8gBUyRUGQ=iQWPvw@mail.gmail.com>
Date:   Fri, 9 Jun 2023 15:58:44 -0700
From:   Chun-Tse Shao <ctshao@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-kernel@...r.kernel.org, yuzhao@...gle.com,
        oliver.upton@...ux.dev, 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 Fri, Jun 9, 2023 at 12:44 AM Marc Zyngier <maz@...nel.org> wrote:
>
> On Thu, 08 Jun 2023 23:05:41 +0100,
> Chun-Tse Shao <ctshao@...gle.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.

Thanks for the suggestion, we are currently working on the performance
test in parallel and will update after gathering more data.

>
> 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?

Sure, I will start internal meeting as needed.

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

Thanks,
CT