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From: Peter Zijlstra <peterz@infradead.org>
To: Like Xu <like.xu.linux@gmail.com>
Cc: Song Liu <songliubraving@fb.com>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	like.xu@linux.intel.com
Subject: Re: bpf_get_branch_snapshot on qemu-kvm
Date: Wed, 29 Sep 2021 14:26:07 +0200	[thread overview]
Message-ID: <YVRbX6vBgz+wYzZK@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <d75f6a9a-dbb3-c725-c001-ec9bdd55173f@gmail.com>

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.


  reply	other threads:[~2021-09-29 12:26 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-29  0:04 bpf_get_branch_snapshot on qemu-kvm Song Liu
2021-09-29  7:35 ` Peter Zijlstra
2021-09-29 12:05   ` Like Xu
2021-09-29 12:26     ` Peter Zijlstra [this message]
2021-09-29 14:42       ` Song Liu
2021-09-29 15:59         ` Peter Zijlstra
2021-09-29 16:35           ` Liang, Kan
2021-09-29 20:05             ` Song Liu
2021-10-08  3:34               ` Like Xu
2021-10-08  5:46                 ` Song Liu
2021-10-08  6:36                   ` Like Xu
2021-10-08 17:08                     ` Song Liu
2021-10-09  9:03                       ` Like Xu
2021-10-26  7:09                         ` Song Liu
2021-10-28  3:09                           ` Like Xu
2021-10-29 18:09                             ` Song Liu
2021-09-29 23:20           ` Andi Kleen
2021-09-29 14:39     ` Song Liu

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