Re: [PATCH 03/12] perf/x86/intel: Add perf core PMU support for Sapphire Rapids

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jan 26, 2021 at 10:44:17AM -0500, Liang, Kan wrote:
> 
> 
> On 1/26/2021 9:44 AM, Peter Zijlstra wrote:
> > On Tue, Jan 19, 2021 at 12:38:22PM -0800, kan.liang@linux.intel.com wrote:
> > > @@ -3671,6 +3853,31 @@ static int intel_pmu_hw_config(struct perf_event *event)
> > >   		}
> > >   	}
> > > +	/*
> > > +	 * To retrieve complete Memory Info of the load latency event, an
> > > +	 * auxiliary event has to be enabled simultaneously. Add a check for
> > > +	 * the load latency event.
> > > +	 *
> > > +	 * In a group, the auxiliary event must be in front of the load latency
> > > +	 * event. The rule is to simplify the implementation of the check.
> > > +	 * That's because perf cannot have a complete group at the moment.
> > > +	 */
> > > +	if (x86_pmu.flags & PMU_FL_MEM_LOADS_AUX &&
> > > +	    (event->attr.sample_type & PERF_SAMPLE_DATA_SRC) &&
> > > +	    is_mem_loads_event(event)) {
> > > +		struct perf_event *leader = event->group_leader;
> > > +		struct perf_event *sibling = NULL;
> > > +
> > > +		if (!is_mem_loads_aux_event(leader)) {
> > > +			for_each_sibling_event(sibling, leader) {
> > > +				if (is_mem_loads_aux_event(sibling))
> > > +					break;
> > > +			}
> > > +			if (list_entry_is_head(sibling, &leader->sibling_list, sibling_list))
> > > +				return -ENODATA;
> > > +		}
> > > +	}
> > > +
> > >   	if (!(event->attr.config & ARCH_PERFMON_EVENTSEL_ANY))
> > >   		return 0;
> > 
> > I have vague memories of this getting mentioned in a call at some point.
> > Pretend I don't know anything and tell me more.
> > 
> 
> Adding the auxiliary event is for the new data source fields, data block &
> address block. If perf only samples the load latency event, the value of the
> data block & address block fields in a sample is not correct. To get the
> correct value, we have to sample both the auxiliary event and the load
> latency together on SPR. So I add the check in the kernel. I also modify the
> perf mem in the perf tool accordingly.

This is an active work around for a chip defect right? Something we're
normally have an errata for. Can we call it that?