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

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Date:   Thu, 6 Jan 2022 11:29:19 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.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 6/1/2022 6:51 am, Jim Mattson wrote:
> 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. :-)

Uh, thanks for your attention.

> 
> 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.
> 

The emulation based on guest TSC semantics w/ low precision may work it out.
TBH, I still haven't given up on the idea of a pass-through approach.