On Mon, Oct 14, 2024 at 11:40:21AM -0400, Liang, Kan wrote:
> >> @@ -11842,7 +11860,21 @@ int perf_pmu_register(struct pmu *pmu, const char *name, int type)
> >> if (!pmu->event_idx)
> >> pmu->event_idx = perf_event_idx_default;
> >>
> >> - list_add_rcu(&pmu->entry, &pmus);
> >> + /*
> >> + * Initialize passthru_pmu with the core pmu that has
> >> + * PERF_PMU_CAP_PASSTHROUGH_VPMU capability.
> >> + */
> >> + if (pmu->capabilities & PERF_PMU_CAP_PASSTHROUGH_VPMU) {
> >> + if (!passthru_pmu)
> >> + passthru_pmu = pmu;
> >> +
> >> + if (WARN_ONCE(passthru_pmu != pmu, "Only one passthrough PMU is supported\n")) {
> >> + ret = -EINVAL;
> >> + goto free_dev;
> >
> > Why impose this limit? Changelog also fails to explain this.
>
> Because the passthru_pmu is global variable. If there are two or more
> PMUs with the PERF_PMU_CAP_PASSTHROUGH_VPMU, the former one will be
> implicitly overwritten if without the check.
That is not the question; but is has been answered elsewhere. For some
reason you thought the srcu+list thing was expensive.
