linux-kernel - Re: [PATCH v4 3/7] perf/x86/amd/core: Detect PerfMonV2 support

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Message-ID: <YnkTFRNpCykEndsk@hirez.programming.kicks-ass.net>
Date:   Mon, 9 May 2022 15:11:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sandipan Das <sandipan.das@....com>
Cc:     Like Xu <like.xu.linux@...il.com>, 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, eranian@...gle.com,
        puwen@...on.cn, ananth.narayan@....com, ravi.bangoria@....com,
        santosh.shukla@....com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-perf-users@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v4 3/7] perf/x86/amd/core: Detect PerfMonV2 support

On Mon, May 09, 2022 at 06:38:19PM +0530, Sandipan Das wrote:
> 
> On 5/9/2022 6:31 PM, Like Xu wrote:
> > On 27/4/2022 7:31 pm, Sandipan Das wrote:
> > 
> >>       x86_pmu.num_counters    = AMD64_NUM_COUNTERS_CORE;
> > 
> > Thus boot_cpu_has(X86_FEATURE_PERFCTR_CORE) is true.
> > 
> >> +
> >> +    /* Check for Performance Monitoring v2 support */
> >> +    if (boot_cpu_has(X86_FEATURE_PERFMON_V2)) {
> >> +        /* Update PMU version for later usage */
> >> +        x86_pmu.version = 2;
> > 
> > Is it safe to assume that once AMD CPU has the PERFMON_V2 (or further) bit,
> > 
> > it must also have the PERFCTR_CORE bit set ?
> > 
> 
> Yes, always. There won't be a case where PERFCTR_CORE is absent
> but PERFMON_V2 is present.

Let me introduce you to this dodgy virt stuff :-) Best put a sanity
check on it.