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

[Ingo Molnar]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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