On Wed, Sep 29, 2021 at 02:42:27PM +0000, Song Liu wrote:

Hi Peter, 

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On Sep 29, 2021, at 5:26 AM, Peter Zijlstra [off-list ref] wrote:

On Wed, Sep 29, 2021 at 08:05:24PM +0800, Like Xu wrote:

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On 29/9/2021 3:35 pm, Peter Zijlstra wrote:

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[  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

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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.