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

[Peter Zijlstra]

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?