[PATCH 5/5] perf: Correct perf sampling with guest VMs

[Mark Rutland]

On Wed, Sep 04, 2024 at 08:41:33PM +0000, Colton Lewis wrote:
> Previously any PMU overflow interrupt that fired while a VCPU was
> loaded was recorded as a guest event whether it truly was or not. This
> resulted in nonsense perf recordings that did not honor
> perf_event_attr.exclude_guest and recorded guest IPs where it should
> have recorded host IPs.
> 
> Reorganize that plumbing to record perf events correctly even when
> VCPUs are loaded.

It'd be good if we could make that last bit a little more explicit,
e.g.

  Rework the sampling logic to only record guest samples for events with
  exclude_guest clear. This way any host-only events with exclude_guest
  set will never see unexpected guest samples. The behaviour of events
  with exclude_guest clear is unchanged.

[...]

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 4384f6c49930..e1a66c9c3773 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6915,13 +6915,26 @@ void perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
>  EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
>  #endif
>  
> -unsigned long perf_misc_flags(unsigned long pt_regs *regs)
> +static bool is_guest_event(struct perf_event *event)
>  {
> +	return !event->attr.exclude_guest && perf_guest_state();
> +}

Could we name this something like "should_sample_guest()"? Calling this
"is_guest_event()" makes it should like it's checking a static property
of the event (and not other conditions like perf_guest_state()).

Otherwise this all looks reasonable to me, modulo Ingo's comments. I'll
happily test a v2 once those have been addressed.

Mark.