linux-kernel - Re: [PATCH v4 23/38] KVM: x86/pmu: Configure the interception of PMU MSRs

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Message-ID: <aCY7JkJA25D2HfXF@google.com>
Date: Thu, 15 May 2025 12:06:14 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Mingwei Zhang <mizhang@...gle.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Liang@...gle.com, 
	Kan <kan.liang@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Yongwei Ma <yongwei.ma@...el.com>, Xiong Zhang <xiong.y.zhang@...ux.intel.com>, 
	Jim Mattson <jmattson@...gle.com>, Sandipan Das <sandipan.das@....com>, 
	Zide Chen <zide.chen@...el.com>, Eranian Stephane <eranian@...gle.com>, 
	Shukla Manali <Manali.Shukla@....com>, Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 23/38] KVM: x86/pmu: Configure the interception of PMU MSRs

On Thu, May 15, 2025, Dapeng Mi wrote:
> On 5/15/2025 8:41 AM, Sean Christopherson wrote:
> >> +	if (kvm_mediated_pmu_enabled(vcpu) && kvm_pmu_has_perf_global_ctrl(pmu) &&
> > Just require the guest to have PERF_GLOBAL_CTRL, I don't see any reason to support
> > v1 PMUs.  It adds complexity and weirdness, and I can't imagine there's a use case.

I take that back, there absolutely are use cases, especially for AMD.  Any VM
shape that exists today should be compatible with the mediated PMU.  And I was
dead wrong about adding complexity; KVM already needs to intercept GLOBAL_CTRL
if the guest has fewer PMCs than hardware, so incorporating this check is all of
two lines of code.