linux-kernel - Re: [PATCH] x86/perf: Fix guest_get_msrs static call if there is no PMU

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Message-ID: <b209d39c-8c0d-93ce-c81d-be2dfea33ad6@intel.com>
Date:   Mon, 8 Mar 2021 20:01:52 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Like Xu <like.xu@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>, kvm@...r.kernel.org,
        Thomas Gleixner
         "(x86/pti/timer/core/smp/irq/perf/efi/locking/ras/objtool)"
         "(x86@...nel.org)" <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] x86/perf: Fix guest_get_msrs static call if there is no
 PMU

On 2021/3/8 16:53, Peter Zijlstra wrote:
> Still, it calling atomic_switch_perf_msrs() and
> intel_pmu_lbr_is_enabled() when there isn't a PMU at all is of course, a
> complete waste of cycles.

This suggestion is reminiscent of a sad regression of optimizing it:

https://lore.kernel.org/kvm/20200619094046.654019-1-vkuznets@redhat.com/
https://lore.kernel.org/kvm/20210209225653.1393771-1-jmattson@google.com/