Re: [PATCH 1/4] perf/x86/intel: Don't write PEBS_ENABLED on host<=>guest xfers if CPU has isolation

[Namhyung Kim]

On Thu, Apr 16, 2026 at 12:38:49PM -0700, Sean Christopherson wrote:
> On Thu, Apr 16, 2026, Namhyung Kim wrote:
> > > +	/*
> > > +	 * Disable counters where the guest PMC is different than the host PMC
> > > +	 * being used on behalf of the guest, as the PEBS record includes
> > > +	 * PERF_GLOBAL_STATUS, i.e. the guest will see overflow status for the
> > > +	 * wrong counter(s).  Similarly, disallow PEBS in the guest if the host
> > > +	 * is using PEBS, to avoid bleeding host state into PEBS records.
> > > +	 */
> > > +	guest_pebs_mask &= kvm_pmu->pebs_enable & ~kvm_pmu->host_cross_mapped_mask;
> > > +	if (pebs_mask & ~cpuc->intel_ctrl_guest_mask)
> > > +		guest_pebs_mask = 0;
> > >  
> > > +	/*
> > > +	 * Do NOT mess with PEBS_ENABLED.  As above, disabling counters via
> > > +	 * PERF_GLOBAL_CTRL is sufficient, and loading a stale PEBS_ENABLED,
> > > +	 * e.g. on VM-Exit, can put the system in a bad state.  Simply enable
> > > +	 * counters in PERF_GLOBAL_CTRL, as perf load PEBS_ENABLED with the
> > > +	 * full value, i.e. perf *also* relies on PERF_GLOBAL_CTRL.
> > > +	 */
> > > +	arr[global_ctrl].guest |= guest_pebs_mask;
> > 
> > I was confused by the earlier comment in the funcion that says it is not
> > enough to disable counters but I've realized it's only for the case PEBS
> > isolation is not supported by CPU/ucode.
> 
> Yeah, me too, more than once. :-/
> 
> > I think it's ok for disabling guest PEBS, but I'm curious if there's a
> > case to enable PEBS only in guest and it'd be handled correctly.
> 
> Yep, if PEBS is being virtualized for the guest, unless the host is also profiling,
> then PEBS will be active for the guest but not the host.  KVM tests for PEBS pass,
> and while they aren't exactly comprehensive, they should detect outright breakage.

In that case, wouldn't it need to update PEBS_ENABLED here?

Thanks,
Namhyung