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

[Colton Lewis]

Mark Rutland <mark.rutland@xxxxxxx> writes:

> 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.

> [...]

Done

> > 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.

Done

> Mark.