netdev - Re: bpf_get_branch_snapshot on qemu-kvm

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Message-ID: <C6DF009D-161A-4B17-88AE-3982DD6F22A2@fb.com>
Date:   Wed, 29 Sep 2021 14:42:27 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
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>
Subject: Re: bpf_get_branch_snapshot on qemu-kvm

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?

Thanks,
Song

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