linux-kernel - Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics event

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Date:   Wed, 27 Sep 2023 13:33:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Like Xu <likexu@...cent.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>, kvm@...r.kernel.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhang Xiong <xiong.y.zhang@...el.com>,
        Lv Zhiyuan <zhiyuan.lv@...el.com>,
        Yang Weijiang <weijiang.yang@...el.com>,
        Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics
 event

On Wed, Sep 27, 2023 at 11:31:18AM +0800, Dapeng Mi wrote:
> When guest wants to use PERF_METRICS MSR, a virtual metrics event needs
> to be created in the perf subsystem so that the guest can have exclusive
> ownership of the PERF_METRICS MSR.

Urgh, can someone please remind me how all that is supposed to work
again? The guest is just a task that wants the event. If the
host creates a CPU event, then that gets scheduled with higher priority
and the task looses out, no joy.

So you cannot guarantee the guest gets anything.

That is, I remember we've had this exact problem before, but I keep
forgetting how this all is supposed to work. I don't use this virt stuff
(and every time I try qemu arguments defeat me and I give up in
disgust).