On Wed, Jun 09, 2021 at 05:20:53PM +0300, Adrian Hunter wrote:

+static void intel_pmu_report_aux_output_id(struct perf_event *event)
+{
+	struct hw_perf_event *hwc = &event->hw;
+
+	/*
+	 * So long as all PEBS-via-PT events for a recording session are
+	 * scheduled together, then only changes to hwc->idx need be reported.
+	 */
+	if (hwc->idx != hwc->idx_reported) {
+		hwc->idx_reported = hwc->idx;
+		perf_report_aux_output_id(event, hwc->idx);
+	}
+}

AFAICT you want a callback in x86_assign_hw_event(), is that so?