linux-kernel - Re: [PATCH 4/7] perf/x86/amd/core: Detect available counters

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Message-ID: <YjMcVVkG+kOZgtYB@hirez.programming.kicks-ass.net>
Date:   Thu, 17 Mar 2022 12:32:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sandipan Das <sandipan.das@....com>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        x86@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, namhyung@...nel.org,
        jolsa@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        pbonzini@...hat.com, jmattson@...gle.com, like.xu.linux@...il.com,
        eranian@...gle.com, ananth.narayan@....com, ravi.bangoria@....com,
        santosh.shukla@....com
Subject: Re: [PATCH 4/7] perf/x86/amd/core: Detect available counters

On Thu, Mar 17, 2022 at 11:58:33AM +0530, Sandipan Das wrote:
> If AMD Performance Monitoring Version 2 (PerfMonV2) is
> supported, use CPUID Fn80000022[EBX] to detect the number
> of Core PMCs. This offers more flexibility if the counts
> change across processor families.
> 
> Signed-off-by: Sandipan Das <sandipan.das@....com>
> ---
>  arch/x86/events/amd/core.c        | 5 +++++
>  arch/x86/include/asm/perf_event.h | 8 ++++++++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> index a074af97faa9..05d79afe5173 100644
> --- a/arch/x86/events/amd/core.c
> +++ b/arch/x86/events/amd/core.c
> @@ -980,9 +980,14 @@ static int __init amd_core_pmu_init(void)
>  
>  	/* Check for Performance Monitoring v2 support */
>  	if (boot_cpu_has(X86_FEATURE_PERFMON_V2)) {
> +		int ebx = cpuid_ebx(EXT_PERFMON_DEBUG_FEATURES);
> +
>  		/* Update PMU version for later usage */
>  		x86_pmu.version = 2;
>  
> +		/* Find the number of available Core PMCs */
> +		x86_pmu.num_counters = EXT_PERFMON_DEBUG_NUM_CORE_PMC(ebx);
> +
>  		amd_pmu_global_cntr_mask = (1ULL << x86_pmu.num_counters) - 1;
>  	}

I prefer using unions like cpuid10_ebx. Such much easier to read than
all this shouting.