linux-kernel - Re: [PATCH v4 04/38] perf: Add a EVENT

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Message-ID: <aCUeh6KxgjLxd-MK@google.com>
Date: Wed, 14 May 2025 15:51:51 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Mingwei Zhang <mizhang@...gle.com>
Cc: 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>, 
	Dapeng Mi <dapeng1.mi@...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 04/38] perf: Add a EVENT_GUEST flag

On Mon, Mar 24, 2025, Mingwei Zhang wrote:
> +{
> +	if (event->attr.exclude_guest && __this_cpu_read(perf_in_guest)) {

My vote is for s/perf_in_guest/guest_ctx_loaded, because "perf in guest" doesn't
accurately describe just the mediated PMU case.  E.g. perf itself is running in
KVM guests when using an emulated vPMU, or no vPMU at all.

And with a Kconfig to guard the mediated PMU, this check (and others) can be
elided at compile time for architectures that don't support a mediated PMU (or
if KVM is disabled).

> +		/*
> +		 * (now + times[total].offset) - (now + times[guest].offset) :=
> +		 * times[total].offset - times[guest].offset
> +		 */
> +		return READ_ONCE(times[T_TOTAL].offset) - READ_ONCE(times[T_GUEST].offset);
> +	}
> +
> +	return now + READ_ONCE(times[T_TOTAL].offset);
> +}
> +