linux-kernel - Re: [PATCH v4 01/38] perf: Support get/put mediated PMU interfaces

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Message-ID: <60ca046d-b706-47ff-bbdb-9e6646af5250@linux.intel.com>
Date: Thu, 15 May 2025 09:31:01 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Mingwei Zhang <mizhang@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, Liang@...gle.com,
 Kan <kan.liang@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Yongwei Ma <yongwei.ma@...el.com>,
 Xiong Zhang <xiong.y.zhang@...ux.intel.com>,
 Jim Mattson <jmattson@...gle.com>, Sandipan Das <sandipan.das@....com>,
 Zide Chen <zide.chen@...el.com>, Eranian Stephane <eranian@...gle.com>,
 Shukla Manali <Manali.Shukla@....com>,
 Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 01/38] perf: Support get/put mediated PMU interfaces

On 5/15/2025 6:48 AM, Sean Christopherson wrote:
> On Mon, Mar 24, 2025, Mingwei Zhang wrote:
>> +/*
>> + * Currently invoked at VM creation to
>> + * - Check whether there are existing !exclude_guest events of PMU with
>> + *   PERF_PMU_CAP_MEDIATED_VPMU
>> + * - Set nr_mediated_pmu_vms to prevent !exclude_guest event creation on
>> + *   PMUs with PERF_PMU_CAP_MEDIATED_VPMU
>> + *
>> + * No impact for the PMU without PERF_PMU_CAP_MEDIATED_VPMU. The perf
>> + * still owns all the PMU resources.
>> + */
>> +int perf_get_mediated_pmu(void)
>> +{
>> +	guard(mutex)(&perf_mediated_pmu_mutex);
>> +	if (atomic_inc_not_zero(&nr_mediated_pmu_vms))
>> +		return 0;
>> +
>> +	if (atomic_read(&nr_include_guest_events))
>> +		return -EBUSY;
>> +
>> +	atomic_inc(&nr_mediated_pmu_vms);
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(perf_get_mediated_pmu);
> IMO, all of the mediated PMU logic should be guarded with a Kconfig.  I strongly
> suspect KVM x86 will be the only user for the foreseeable, e.g. arm64 is trending
> toward a partioned PMU approach, and subjecting other architectures to the (minor)
> overhead associated with e.g. nr_mediated_pmu_vms seems pointless.  The other
> nicety is that it helps encapsulate the mediated PMU code, which for those of us
> that haven't been living and breathing this for the last few months, is immensely
> helpful.

I'm fine with this.

>
>> +void perf_put_mediated_pmu(void)
> To avoid confusion with perf_put_guest_context() in future patches, I think it
> makes sense to go with something like perf_{create,release}_mediated_pmu().  I
> actually like the get/put terminology in isolation, but they look weird side-by-side.

Agree.