Re: [Patch v2 7/9] perf/x86/intel: Drop fixed-counter PEBS constraints for baseline PEBS

[Peter Zijlstra]

On Wed, Jun 10, 2026 at 04:50:45PM +0800, Mi, Dapeng wrote:
> 
> On 6/10/2026 4:20 PM, Peter Zijlstra wrote:
> > On Tue, Jun 09, 2026 at 01:02:20PM +0800, Dapeng Mi wrote:
> >> On SPR guests where pebs_baseline is not advertised, running:
> >>
> >> $ ./perf record -e cpu/event=0x00,umask=0x01,i\
> >> 	 name=INST_RETIRED.PREC_DIST/p -c 10000 sleep 1
> >>
> >> can trigger:
> >>
> >> unchecked MSR access error: WRMSR to 0x3f1 ... in\
> >> 	 intel_pmu_pebs_enable_all()
> >>
> >> Root cause:
> >> SPR-specific PEBS constraints allow fixed-counter scheduling,
> >> for example INST_RETIRED.PREC_DIST on fixed counter 0. In guests without
> >> pebs_baseline, KVM does not support PEBS sampling on fixed counters,
> >> so enabling such events reaches an invalid MSR programming path.
> >>
> >> Fix:
> >> Drop fixed-counter entries from the PEBS constraint table. Without
> >> pebs_baseline, those fixed-counter PEBS events now resolve to empty
> >> constraints and are not scheduled/enabled, avoiding the warning and the
> >> broken guest PEBS path.
> >>
> >> This is safe because, in pebs_baseline-capable cases, PEBS constraint
> >> lookup already falls back to non-PEBS constraints when needed, and
> >> fixed-counter constraints are effectively shared there.
> > I am confused, this works outside of KVM? (It appears to work fine on my
> > spr).. so removing this to fix some guest only issue seems wrong.
> 
> The reason that it works on bare metal is currently the constraint lookup
> would fallback into non-PEBS constraints if there is no matched entry in
> the PEBS constraints as long as the PEBS supports sampling on all counters
> including fixed counters (what the flag "PMU_FL_PEBS_ALL" indicates), like
> the below code shows.
> 
> ```

Why the markdown nonsense? You're not an LLM, right? And you all know
I'm reading this in mutt. Its just visual noise.

Anyway, please clarify the Changelog.