linux-kernel - Re: [RFC PATCH 00/22] KVM: x86: Virtualize IA32_APERF and IA32

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Message-ID: <465c927c-88f0-45fc-9178-4c6981f82fd9@amd.com>
Date: Thu, 5 Dec 2024 14:29:55 +0530
From: Nikunj A Dadhania <nikunj@....com>
To: Mingwei Zhang <mizhang@...gle.com>,
 Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>, Huang Rui <ray.huang@....com>,
 "Gautham R. Shenoy" <gautham.shenoy@....com>,
 Mario Limonciello <mario.limonciello@....com>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
 Len Brown <lenb@...nel.org>
Cc: "H. Peter Anvin" <hpa@...or.com>, Perry Yuan <perry.yuan@....com>,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
 Jim Mattson <jmattson@...gle.com>
Subject: Re: [RFC PATCH 00/22] KVM: x86: Virtualize IA32_APERF and IA32_MPERF
 MSRs

On 11/22/2024 12:22 AM, Mingwei Zhang wrote:
> Linux guests read IA32_APERF and IA32_MPERF on every scheduler tick
> (250 Hz by default) to measure their effective CPU frequency. To avoid
> the overhead of intercepting these frequent MSR reads, allow the guest
> to read them directly by loading guest values into the hardware MSRs.
> 
> These MSRs are continuously running counters whose values must be
> carefully tracked during all vCPU state transitions:
> - Guest IA32_APERF advances only during guest execution
> - Guest IA32_MPERF advances at the TSC frequency whenever the vCPU is
>   in C0 state, even when not actively running

Any particular reason to treat APERF and MPERF differently?

AFAIU, APERF and MPERF architecturally will count when the CPU is in C0 state.
MPERF counting at constant frequency and the APERF counting at a variable
frequency. Shouldn't we treat APERF and MPERF equal and keep on counting in C0
state and even when "not actively running" ?

Can you clarify what do you mean by "not actively running"?

Regards
Nikunj