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From:   "Wang, Wei W" <wei.w.wang@intel.com>
To:     "Liang, Kan" <kan.liang@linux.intel.com>,
        "Li, Xiaoyao" <xiaoyao.li@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        "Namhyung Kim" <namhyung@kernel.org>,
        "Christopherson,, Sean" <seanjc@google.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: RE: [RFC PATCH v2 2/3] perf/x86/intel/pt: Introduce and export
 pt_get_curr_event()
Thread-Topic: [RFC PATCH v2 2/3] perf/x86/intel/pt: Introduce and export
 pt_get_curr_event()
Thread-Index: AQHYzdmZiDfxfKVenUORyjhoUz2tZ63rY3WAgAAA7VA=
Date:   Thu, 22 Sep 2022 12:58:49 +0000
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 <175b518c-d202-644e-a3a7-67e877852548@linux.intel.com>
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On Thursday, September 22, 2022 8:34 PM, Liang, Kan wrote:
> > To solve the problem, introduce and export pt_get_curr_event() for KVM
> > to get current pt event.
>=20
> I don't think the current pt event is created by KVM. IIUC, the patch bas=
ically
> expose the event created by the other user to KVM. That doesn't sounds
> correct.

Yes, that's the host PT event running on the current CPU. Not created by KV=
M.

>=20
> I think it should be perf's responsibility to decide which events should =
be
> disabled, and which MSRs should be switched. Because only perf can see al=
l the
> events. The users should only see the events they created.

For other pmu cases, yes. For PT, its management is simpler than other pmu
resources and PT PMU is much simpler. It doesn't have a scheduler to manage
events.

I think the usage here is similar to the CPU thread scheduling case. When w=
e have
CPUs isolated from the CPU scheduler, i.e. no scheduler for those CPUs, bas=
ically
we rely on users to ping tasks to those CPUs (e.g. taskset).

For the commit log, probably we don't need those KVM details here. Simplify=
 a bit:

Add a function to expose the current running PT event to users. One usage i=
s 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.