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

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Message-ID: <DS0PR11MB63730CE4DAE000E338F092A0DC559@DS0PR11MB6373.namprd11.prod.outlook.com>
Date:   Tue, 27 Sep 2022 14:27:00 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Jim Mattson <jmattson@...gle.com>
CC:     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 Tuesday, September 27, 2022 2:09 AM, Liang, Kan wrote:
> From my understanding of the host-guest mode, the host PT event never
> traces the guest no matter whether the guest enables PT. So when VM-entry,
> there is only a guest PT event or no event. If so, I think the perf tool should
> warn the user if they try to create a host event with !exclude_guest, since the
> host event never traces a guest.

Probably not from the perf side. It's actually an issue of the KVM's pt_mode:
How can KVM prevent host from profiling the guest when in host-guest mode?
Just a warning from the perf side wouldn't be enough. I think that's a wrong
assumption from KVM side. I had a plan to fix this one. Exposing the host event
can tell KVM if the host is profiling the guest or not (i.e. host-guest is allowed).