linux-kernel - Re: [PATCH 03/20] perf/x86/intel: Parse CPUID archPerfmonExt leaves for non-hybrid CPUs

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Message-ID: <20250127164406.GN16742@noisy.programming.kicks-ass.net>
Date: Mon, 27 Jan 2025 17:44:06 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Liang, Kan" <kan.liang@...ux.intel.com>
Cc: Andi Kleen <ak@...ux.intel.com>, Dapeng Mi <dapeng1.mi@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Eranian Stephane <eranian@...gle.com>, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org, Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [PATCH 03/20] perf/x86/intel: Parse CPUID archPerfmonExt leaves
 for non-hybrid CPUs

On Mon, Jan 27, 2025 at 10:19:34AM -0500, Liang, Kan wrote:
> 
> 
> On 2025-01-23 1:58 p.m., Andi Kleen wrote:
> >> +	/*
> >> +	 * The archPerfmonExt (0x23) includes an enhanced enumeration of
> >> +	 * PMU architectural features with a per-core view. For non-hybrid,
> >> +	 * each core has the same PMU capabilities. It's good enough to
> >> +	 * update the x86_pmu from the booting CPU. For hybrid, the x86_pmu
> >> +	 * is used to keep the common capabilities. Still keep the values
> >> +	 * from the leaf 0xa. The core specific update will be done later
> >> +	 * when a new type is online.
> >> +	 */
> >> +	if (!is_hybrid() && boot_cpu_has(X86_FEATURE_ARCH_PERFMON_EXT))
> >> +		update_pmu_cap(NULL);
> > 
> > It seems ugly to have these different code paths. Couldn't non hybrid
> > use x86_pmu in the same way? I assume it would be a larger patch.
> 
> The current non-hybrid is initialized in the intel_pmu_init(). But some
> of the initialization code for the hybrid is in the
> intel_pmu_cpu_starting(). Yes, it's better to move it together. It
> should be a larger patch. Since it's impacted other features, a separate
> patch set should be required.

IIRC the problem was that there were SKUs with the same FMS that were
both hybrid and non-hybrid and we wouldn't know until we brought up the
CPUs.

Thomas rewrote the topology bits since, so maybe we can do beter these
days.