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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Mon, 2 Oct 2023 15:30:07 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Sean Christopherson <seanjc@google.com>,
        Dapeng Mi <dapeng1.mi@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Kan Liang <kan.liang@linux.intel.com>,
        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>
Subject: Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics
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* Peter Zijlstra <peterz@infradead.org> wrote:

> 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. 

Not just a choice of laziness, but it will clearly be forced upon users
by external entities:

   "Pass ownership of the PMU to the guest and have no host PMU, or you
    won't have sane guest PMU support at all. If you disagree, please open
    a support ticket, which we'll ignore."

The host OS shouldn't offer facilities that severely limit its own capabilities,
when there's a better solution. We don't give the FPU to apps exclusively either,
it would be insanely stupid for a platform to do that.

Thanks,

	Ingo