netdev - Re: bpf_get_branch_snapshot on qemu-kvm

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Date:   Wed, 29 Sep 2021 17:59:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     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>,
        "Liang, Kan" <kan.liang@...el.com>,
        Andi Kleen <andi@...stfloor.org>
Subject: Re: bpf_get_branch_snapshot on qemu-kvm

On Wed, Sep 29, 2021 at 02:42:27PM +0000, Song Liu wrote:
> Hi Peter, 
> 
> > On Sep 29, 2021, at 5:26 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > On Wed, Sep 29, 2021 at 08:05:24PM +0800, Like Xu wrote:
> >> On 29/9/2021 3:35 pm, Peter Zijlstra wrote:
> > 
> >>>> [  139.494159] unchecked MSR access error: WRMSR to 0x3f1 (tried to write 0x0000000000000000) at rIP: 0xffffffff81011a8b (intel_pmu_snapshot_branch_stack+0x3b/0xd0)
> >> 
> >> Uh, it uses a PEBS counter to sample or count, which is not yet upstream but
> >> should be soon.
> > 
> > Ooh that's PEBS_ENABLE
> > 
> >> Song, can you try to fix bpf_get_branch_snapshot on a normal PMC counter,
> >> or where is the src for bpf_get_branch_snapshot? I am more than happy to help.
> > 
> > Nah, all that code wants to do is disable PEBS... and virt being virt,
> > it's all sorts of weird with f/m/s :/
> > 
> > I so hate all that. So there's two solutions:
> > 
> > - 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.