linux-kernel - Re: [PATCH 3/7] perf/x86/amd/core: Detect PerfMonV2 support

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Date:   Thu, 17 Mar 2022 12:27:24 +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 3/7] perf/x86/amd/core: Detect PerfMonV2 support

On Thu, Mar 17, 2022 at 11:58:32AM +0530, Sandipan Das wrote:
> AMD Performance Monitoring Version 2 (PerfMonV2) introduces
> some new Core PMU features such as detection of the number
> of available PMCs and managing PMCs using global registers
> namely, PerfCntrGlobalCtl and PerfCntrGlobalStatus.
> 
> Clearing PerfCntrGlobalCtl and PerfCntrGlobalStatus ensures
> that all PMCs are inactive and have no pending overflows
> when CPUs are onlined or offlined.
> 
> The PMU version (x86_pmu.version) now indicates PerfMonV2
> support and will be used to bypass the new features on
> unsupported processors.
> 
> Signed-off-by: Sandipan Das <sandipan.das@....com>
> ---

> +static void amd_pmu_cpu_reset(int cpu)
> +{
> +	if (x86_pmu.version < 2)
> +		return;
> +
> +	/* Clear enable bits i.e. PerfCntrGlobalCtl.PerfCntrEn */
> +	wrmsrl_on_cpu(cpu, MSR_AMD64_PERF_CNTR_GLOBAL_CTL, 0);
> +
> +	/* Clear overflow bits i.e. PerfCntrGLobalStatus.PerfCntrOvfl */
> +	wrmsrl_on_cpu(cpu, MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_CLR,
> +		      amd_pmu_global_cntr_mask);
> +}

I think these can be wrmsrl() both starting and dead run on the target
cpu.