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

[Sean Christopherson]

On Wed, Oct 11, 2023, Peter Zijlstra wrote:
> On Wed, Oct 04, 2023 at 02:50:46PM -0700, Sean Christopherson wrote:
> 
> > Thinking about this more, what if we do a blend of KVM's FPU swapping and debug
> > register swapping?
> > 
> >   A. Load guest PMU state in vcpu_enter_guest() after IRQs are disabled
> >   B. Put guest PMU state (and load host state) in vcpu_enter_guest() before IRQs
> >      are enabled, *if and only if* the current CPU has one or perf events that
> >      wants to use the hardware PMU
> >   C. Put guest PMU state at vcpu_put()
> >   D. Add a perf callback that is invoked from IRQ context when perf wants to
> >      configure a new PMU-based events, *before* actually programming the MSRs,
> >      and have KVM's callback put the guest PMU state
> > 
> 
> No real objection, but I would suggest arriving at that solution by
> first building the simple-stupid thing and then making it more
> complicated in additinoal patches. Hmm?

Yeah, for sure.