Re: perf/amd/ibs: [BUG?] ibs_op sampling silently stops on Zen3: MSR_AMD64_IBSOPCTL latched, no NMI delivered

[Ravi Bangoria]

> I found an issue with ibs_op sampling that I can reproduce on my AMD
> Zen3 laptop.  Under full CPU load, sampling PMIs stop arriving on each
> CPU, one after another.  Reading MSR_AMD64_IBSOPCTL (0xc0011033) while
> perf is still running shows the hardware has latched a completed sample
> (IBS_OP_ENABLE=1, IBS_OP_VAL=1, IBS_OP_CUR_CNT frozen) and is waiting
> for an NMI that never arrives.  The CPU never recovers on its own.
> 
> I am really not very familiar with CPU, so I would appreciate any
> guidance on whether this is known and what to test next.

This is a known Zen3 hardware bug. The workaround involves opening a
PMC event alongside the IBS event so that the IBS sample is acknowledged
during the PMC NMI, thereby resuming the stalled IBS instance.

More details in The Revision guide:

  https://bugzilla.kernel.org/attachment.cgi?id=311307
  Erratum 1285 Processor May Fail to Report IBS (Instruction Based
               Sampling) Performance Sample

Thanks,
Ravi