linux-kernel - Re: [PATCH v2] KVM: X86: Emulate APERF/MPERF to report actual vCPU frequency

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Message-ID: <CALMp9eQmO1zS9urH_B8DeoLp30P7Yxxp9qMwavjmoyt_BSC23A@mail.gmail.com>
Date:   Wed, 5 Jan 2022 14:51:44 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Like Xu <likexu@...cent.com>,
        Dongli Cao <caodongli@...gsoft.com>,
        Li RongQing <lirongqing@...du.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "Thomas Gleixner (kernel-recipes.org)" <tglx@...utronix.de>,
        "Borislav Petkov (kernel-recipes.org)" <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v2] KVM: X86: Emulate APERF/MPERF to report actual vCPU frequency

On Thu, Dec 30, 2021 at 11:48 PM Like Xu <like.xu.linux@...il.com> wrote:
>
> On 31/12/2021 9:29 am, Jim Mattson wrote:

> > At sched-in:
> > 1. Save host APERF/MPERF values from the MSRs.
> > 2. Load the "current" guest APERF/MPERF values into the MSRs (if the
> > vCPU configuration allows for unintercepted reads).
> >
> > At sched-out:
> > 1. Calculate the guest APERF/MPERF deltas for use in step 3.
> > 2. Save the "current" guest APERF/MPERF values.
> > 3. "Restore" the host APERF/MPERF values, but add in the deltas from step 1.
> >
> > Without any writes to IA32_MPERF, I would expect these MSRs to be
> > synchronized across all logical processors, and the proposal above
> > would break that synchronization.

I am learning more about IA32_APERF and IA32_MPERF this year. :-)

My worry above is unfounded. These MSRs only increment in C0, so they
are not likely to be synchronized.

This also raises another issue with your original fast-path
implementation: the host MSRs will continue to count while the guest
is halted. However, the guest MSRs should not count while the guest is
halted.