Re: [RFC PATCH v2 2/3] perf/x86/intel/pt: Introduce and export pt_get_curr_event()

[Jim Mattson]

On Mon, Sep 26, 2022 at 9:55 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2022-09-22 10:42 a.m., Wang, Wei W wrote:
> > On Thursday, September 22, 2022 10:10 PM, Peter Zijlstra wrote:
> >> On Thu, Sep 22, 2022 at 01:59:53PM +0000, Wang, Wei W wrote:
> >>> On Thursday, September 22, 2022 9:35 PM, Peter Zijlstra
> >>>> On Thu, Sep 22, 2022 at 12:58:49PM +0000, Wang, Wei W wrote:
> >>>>
> >>>>> Add a function to expose the current running PT event to users.
> >>>>> One usage is in KVM, it needs to get and disable the running host
> >>>>> PT event before VMEnter to the guest and resumes the event after
> >> VMexit to host.
> >>>>
> >>>> You cannot just kill a host event like that. If there is a host
> >>>> event, the guest looses out.
> >>>
> >>> OK. The intention was to pause the event (that only profiles host
> >>> info) when switching to guest, and resume when switching back to host.
> >>
> >> If the even doesn't profile guest context, then yes. If it does profile guest
> >> context, you can't.
> >
> > Seems better to add this one:
>
> If the guest host mode is enabled, I think the PT driver should not
> allow the perf tool to create a host event with !exclude_guest.

While I agree that guest events should generally have priority over
host events, this is not consistent with the way "normal" PMU events
are handled.