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

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Date:   Thu, 22 Sep 2022 13:59:53 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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.

I don't think it is proper to directly disable it via clear MSR_IA32_RTIT_CTL, e.g. there
may be dangling PT bit in the interrupt status register, this invokes the PT interrupt handler
to re-enable it (for more details about the issue please see patch 3 commit).
Would you have any suggestions to pause it? 

Thanks,
Wei