Messages in this thread | | | Date | Mon, 2 Oct 2023 13:57:18 +0200 | From | Peter Zijlstra <> | Subject | Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics event |
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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.
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