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    SubjectRe: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics event

    * 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

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