Hi Peter, 

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?

Thanks,
Song

or

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