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Date:   Mon, 15 Aug 2022 13:51:17 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Like Xu <like.xu.linux@gmail.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Jim Mattson <jmattson@google.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Kan Liang <kan.liang@linux.intel.com>
Subject: Re: [PATCH v2 1/7] perf/x86/core: Update x86_pmu.pebs_capable for
 ICELAKE_{X,D}
Message-ID: <YvozNSvcxet0gX6b@worktop.programming.kicks-ass.net>
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On Mon, Aug 15, 2022 at 05:43:34PM +0800, Like Xu wrote:

> > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > index 2db93498ff71..b42c1beb9924 100644
> > --- a/arch/x86/events/intel/core.c
> > +++ b/arch/x86/events/intel/core.c
> > @@ -5933,7 +5933,6 @@ __init int intel_pmu_init(void)
> >   		x86_pmu.pebs_aliases = NULL;
> >   		x86_pmu.pebs_prec_dist = true;
> >   		x86_pmu.lbr_pt_coexist = true;
> > -		x86_pmu.pebs_capable = ~0ULL;
> >   		x86_pmu.flags |= PMU_FL_HAS_RSP_1;
> >   		x86_pmu.flags |= PMU_FL_PEBS_ALL;
> >   		x86_pmu.get_event_constraints = glp_get_event_constraints;
> > @@ -6291,7 +6290,6 @@ __init int intel_pmu_init(void)
> >   		x86_pmu.pebs_aliases = NULL;
> >   		x86_pmu.pebs_prec_dist = true;
> >   		x86_pmu.pebs_block = true;
> > -		x86_pmu.pebs_capable = ~0ULL;
> >   		x86_pmu.flags |= PMU_FL_HAS_RSP_1;
> >   		x86_pmu.flags |= PMU_FL_NO_HT_SHARING;
> >   		x86_pmu.flags |= PMU_FL_PEBS_ALL;
> > @@ -6337,7 +6335,6 @@ __init int intel_pmu_init(void)
> >   		x86_pmu.pebs_aliases = NULL;
> >   		x86_pmu.pebs_prec_dist = true;
> >   		x86_pmu.pebs_block = true;
> > -		x86_pmu.pebs_capable = ~0ULL;
> >   		x86_pmu.flags |= PMU_FL_HAS_RSP_1;
> >   		x86_pmu.flags |= PMU_FL_NO_HT_SHARING;
> >   		x86_pmu.flags |= PMU_FL_PEBS_ALL;
> > diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> > index ba60427caa6d..e2da643632b9 100644
> > --- a/arch/x86/events/intel/ds.c
> > +++ b/arch/x86/events/intel/ds.c
> > @@ -2258,6 +2258,7 @@ void __init intel_ds_init(void)
> >   			x86_pmu.drain_pebs = intel_pmu_drain_pebs_icl;
> >   			x86_pmu.pebs_record_size = sizeof(struct pebs_basic);
> >   			if (x86_pmu.intel_cap.pebs_baseline) {
> > +				x86_pmu.pebs_capable = ~0ULL;
> 
> The two features of "Extended PEBS (about pebs_capable)" and "Adaptive PEBS
> (about pebs_baseline)"
> are orthogonal, although the two are often supported together.

The SDM explicitly states that PEBS Baseline implies Extended PEBS. See
3-19.8 (April 22 edition).

The question is if there is hardware that has Extended PEBS but doesn't
have Baseline; and I simply don't know and was hoping Kan could find
out.

That said; the above patch can be further improved by also removing the
PMU_FL_PEBS_ALL lines, which is already set by intel_ds_init().

In general though; the point is, we shouldn't be doing the FMS table
thing for discoverable features. Having pebs_capable = ~0 and
PMU_FL_PEBS_ALL on something with BASELINE set is just wrong.