linux-kernel - Re: [RFC PATCH v2 2/3] perf/x86/intel/pt: Introduce and export pt_get_curr

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Date:   Mon, 26 Sep 2022 10:24:03 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     "Wang, Wei W" <wei.w.wang@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Li, Xiaoyao" <xiaoyao.li@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH v2 2/3] perf/x86/intel/pt: Introduce and export pt_get_curr_event()

On Mon, Sep 26, 2022 at 9:55 AM Liang, Kan <kan.liang@...ux.intel.com> 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.