linux-kernel - Re: [RFC PATCH 2/6] kvm: arm64: Add support for get_cur

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Message-ID: <ZC0rhEHTlz6dHKC0@google.com>
Date:   Wed, 5 Apr 2023 08:04:20 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     David Dai <davidai@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <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>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Saravana Kannan <saravanak@...gle.com>,
        kernel-team@...roid.com, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev
Subject: Re: [RFC PATCH 2/6] kvm: arm64: Add support for get_cur_cpufreq
 service

On Thursday 30 Mar 2023 at 15:43:37 (-0700), David Dai wrote:
> This service allows guests to query the host for frequency of the CPU
> that the vCPU is currently running on.

I assume the intention here is to achieve scale invariance in the guest
to ensure its PELT signals represent how much work is actually being
done. If so, it's likely the usage of activity monitors will be superior
for this type of thing as that may allow us to drop the baked-in
assumption about vCPU pinning. IIRC, AMUs v2 (arm64-specific obv) have
extended support for virtualization, so I'd suggest looking into
supporting that first.

And assuming we also want to support this on hardware that don't have
AMUs, or don't have the right virt extensions, then the only thing I can
think of is to have the VMM expose non-architectural AMUs to the guest,
maybe emulated using PMUs. If the guest uses Linux, it'll need to grow
support for non-architectural AMUs which is its own can of worms though.

Thanks,
Quentin