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

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Date:   Mon, 2 Oct 2023 13:57:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Dapeng Mi <dapeng1.mi@...ux.intel.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>,
        Jim Mattson <jmattson@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics
 event

On Fri, Sep 29, 2023 at 03:46:55PM +0000, Sean Christopherson wrote:

> > I will firmly reject anything that takes the PMU away from the host
> > entirely through.
> 
> Why?  What is so wrong with supporting use cases where the platform owner *wants*
> to give up host PMU and NMI watchdog functionality?  If disabling host PMU usage
> were complex, highly invasive, and/or difficult to maintain, then I can understand
> the pushback.  

Because it sucks.

You're forcing people to choose between no host PMU or a slow guest PMU.
And that's simply not a sane choice for most people -- worse it's not a
choice based in technical reality.

It's a choice out of lazyness, disabling host PMU is not a requirement
for pass-through. 

Like I wrote, all we need to do is ensure vCPU tasks will never have a
perf-event scheduled that covers guest mode. Currently this would be
achievable by having event creation for both:

 - CPU events without attr::exclude_guest=1, and
 - task events for vCPU task of interest without attr::exclude_guest=1

error with -EBUSY or something.

This ensures there are no events active for those vCPU tasks at VMENTER
time and you can haz pass-through.