netdev - RE: bpf_get_branch_snapshot on qemu-kvm

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Message-ID: <SJ0PR11MB4814BBE6651FB9F8F05868FBE8A99@SJ0PR11MB4814.namprd11.prod.outlook.com>
Date:   Wed, 29 Sep 2021 16:35:35 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Song Liu <songliubraving@...com>
CC:     Like Xu <like.xu.linux@...il.com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "like.xu@...ux.intel.com" <like.xu@...ux.intel.com>,
        Andi Kleen <andi@...stfloor.org>
Subject: RE: bpf_get_branch_snapshot on qemu-kvm

> > > - get confirmation that clearing GLOBAL_CTRL is suffient to supress
> > >   PEBS, in which case we can simply remove the PEBS_ENABLE clear.
> >
> > How should we confirm this? Can we run some tests for this? Or do we
> > need hardware experts' input for this?
> 
> I'll put it on the list to ask the hardware people when I talk to them next. But
> maybe Kan or Andi know without asking.

If the GLOBAL_CTRL is explicitly disabled, the counters do not count anymore.
It doesn't matter if PEBS is enabled or not. 

See 6c1c07b33eb0 ("perf/x86/intel: Avoid unnecessary PEBS_ENABLE MSR
access in PMI "). We optimized the PMU handler base on it.

Thanks,
Kan