Re: [PATCH] KVM: vPMU: Don't program counter for interrupt-based event sampling w/o lapic_in_kernel

[Paolo Bonzini]

On 25/10/21 18:31, Sean Christopherson wrote:
> > vPMU depends on in-kernel lapic to deliver pmi interrupt, there is a
> > lot of overhead when creating/maintaining perf_event object,
> > locking/unlocking perf_event_ctx etc for vPMU. It silently fails to
> > deliver pmi interrupt if w/o in-kernel lapic currently. Let's not
> > program counter for interrupt-based event sampling w/o in-kernel
> > lapic support to avoid the whole bothering.
>
> This feels all kinds of wrong.  AFAIK, there's no way for KVM to enumerate to
> the guest that the vPMU isn't capable of generating interrupts.  I.e. any setup
> that exposes a vPMU to the guest without an in-kernel local APIC is either
> inherently broken or requires a paravirtualized guest.  I don't think KVM's bugs
> should be optimized.

Yeah, if it simplified the code it would be a different story, but here 
there's even not one but two new checks.

Paolo