netdev - Re: bpf_get_branch_snapshot on qemu-kvm

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Message-ID: <YVRbX6vBgz+wYzZK@hirez.programming.kicks-ass.net>
Date:   Wed, 29 Sep 2021 14:26:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Song Liu <songliubraving@...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
Subject: Re: bpf_get_branch_snapshot on qemu-kvm

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.

or

 - create another 2 intel_pmu_snapshot_{,arch_}branch_stack() copies
   that omit the PEBS thing.