Re: [PATCH v2 1/7] perf/x86/core: Update x86_pmu.pebs_capable for ICELAKE_{X,D}

[Like Xu]

On 15/8/2022 7:51 pm, Peter Zijlstra wrote:
> 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).

Indeed, "IA32_PERF_CAPABILITIES.PEBS_BASELINE [bit 14]: If set, the following is 
true
- Extended PEBS is supported;
- Adaptive PEBS is supported;"

> 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.

In the SDM world, we actually have three PEBS sub-features:

[July 2017] 18.5.4.1 Extended PEBS (first on Goldmont Plus)
[January 2019] 18.9.2 Adaptive PEBS (first on Tremont)
[April 2021] IA32_PERF_CAPABILITIES.PEBS_BASELINE in the Table 2-2. IA-32 
Architectural MSRs

Waiting Kan to confirm the real world.

> 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.