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

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Message-ID: <YyxsnAFYMLn2U9BT@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Sep 2022 16:09:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Wang, Wei W" <wei.w.wang@...el.com>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        "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 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.