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Date:   Mon, 2 Oct 2023 15:50:24 -0400
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Subject: Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics
 event
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To:     Sean Christopherson <seanjc@google.com>,
        Ingo Molnar <mingo@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Dapeng Mi <dapeng1.mi@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Like Xu <likexu@tencent.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>, kvm@vger.kernel.org,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Zhang Xiong <xiong.y.zhang@intel.com>,
        Lv Zhiyuan <zhiyuan.lv@intel.com>,
        Yang Weijiang <weijiang.yang@intel.com>,
        Dapeng Mi <dapeng1.mi@intel.com>,
        Jim Mattson <jmattson@google.com>,
        David Dunn <daviddunn@google.com>,
        Mingwei Zhang <mizhang@google.com>,
        Thomas Gleixner <tglx@linutronix.de>
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 <ZRrF38RGllA04R8o@gmail.com> <ZRroQg6flyGBtZTG@google.com>
From:   "Liang, Kan" <kan.liang@linux.intel.com>
In-Reply-To: <ZRroQg6flyGBtZTG@google.com>
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On 2023-10-02 11:56 a.m., Sean Christopherson wrote:
> I am completely ok with either approach, but I am not ok with being nak'd on both.
> Because unless there's a magical third option lurking, those two options are the
> only ways for KVM to provide a vPMU that meets the requirements for slice-of-hardware
> use cases.

It may be doable to introduce a knob to enable/disable the "CPU-pinned"
capability of perf_event. If the capability is disabled, the KVM doesn't
need to worry about the counters being gone without notification, since
the "task pinned" has the highest priority at the moment.

It should also keeps the flexibility that sometimes the host wants to
profile the guest.

Now, the NMI watchdog is using a "CPU-pinned" event. But I think it can
be replaced by the buddy system, commit 1f423c905a6b
("watchdog/hardlockup: detect hard lockups using secondary (buddy) CPUs")

Thanks,
Kan