linux-kernel - Re: [PATCH 04/12] perf/x86/intel: Support CPUID 10.ECX to disable fixed counters

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Date:   Tue, 26 Jan 2021 16:44:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     acme@...nel.org, mingo@...nel.org, linux-kernel@...r.kernel.org,
        eranian@...gle.com, namhyung@...nel.org, jolsa@...hat.com,
        ak@...ux.intel.com, yao.jin@...ux.intel.com
Subject: Re: [PATCH 04/12] perf/x86/intel: Support CPUID 10.ECX to disable
 fixed counters

On Tue, Jan 19, 2021 at 12:38:23PM -0800, kan.liang@...ux.intel.com wrote:
> @@ -5228,7 +5231,7 @@ __init int intel_pmu_init(void)
>  	 * Check whether the Architectural PerfMon supports
>  	 * Branch Misses Retired hw_event or not.
>  	 */
> -	cpuid(10, &eax.full, &ebx.full, &unused, &edx.full);
> +	cpuid(10, &eax.full, &ebx.full, &fixed_mask, &edx.full);
>  	if (eax.split.mask_length < ARCH_PERFMON_EVENTS_COUNT)
>  		return -ENODEV;
>  
> @@ -5255,8 +5258,16 @@ __init int intel_pmu_init(void)
>  	if (version > 1) {
>  		int assume = 3 * !boot_cpu_has(X86_FEATURE_HYPERVISOR);
>  
> -		x86_pmu.num_counters_fixed =
> -			max((int)edx.split.num_counters_fixed, assume);
> +		if (!fixed_mask) {
> +			x86_pmu.num_counters_fixed =
> +				max((int)edx.split.num_counters_fixed, assume);
> +		} else {
> +			/*
> +			 * The fixed-purpose counters are enumerated in the ECX
> +			 * since V5 perfmon.
> +			 */

But that's not what the code implements.

> +			x86_pmu.num_counters_fixed = fls(fixed_mask);
> +		}
>  	}

What you were looking for is something like this:

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index fe940082d49a..9ad42cb59606 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -4817,6 +4817,13 @@ __init int intel_pmu_init(void)
 
 		x86_pmu.num_counters_fixed =
 			max((int)edx.split.num_counters_fixed, assume);
+
+		if (version >= 5) {
+			/*
+			 * V5 and later provide a fixed counter mask.
+			 */
+			x86_pmu.num_counters_fixed = fls(fixed_mask);
+		}
 	}
 
 	if (boot_cpu_has(X86_FEATURE_PDCM)) {