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From:   "Wang, Wei W" <wei.w.wang@intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
CC:     "Liang, Kan" <kan.liang@linux.intel.com>,
        "Li, Xiaoyao" <xiaoyao.li@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        "Arnaldo Carvalho de Melo" <acme@kernel.org>,
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        "Alexander Shishkin" <alexander.shishkin@linux.intel.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: AQHYzdmZiDfxfKVenUORyjhoUz2tZ63rY3WAgAAA7VCAABAZgIAAAWLQ
Date:   Thu, 22 Sep 2022 13:59:53 +0000
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On Thursday, September 22, 2022 9:35 PM, Peter Zijlstra
> On Thu, Sep 22, 2022 at 12:58:49PM +0000, Wang, Wei W wrote:
>=20
> > 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.
>=20
> You cannot just kill a host event like that. If there is a host event, th=
e guest
> looses out.

OK. The intention was to pause the event (that only profiles host info) whe=
n 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_C=
TL, e.g. there
may be dangling PT bit in the interrupt status register, this invokes the P=
T 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?=20

Thanks,
Wei