linux-kernel - Re: [PATCH v2 1/7] perf/x86/core: Update x86_pmu.pebs_capable for ICELAKE

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Date:   Mon, 15 Aug 2022 20:48:36 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 1/7] perf/x86/core: Update x86_pmu.pebs_capable for
 ICELAKE_{X,D}

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.