perf: arm_pmuv3: Avoid assigning fixed cycle counter with threshold

If the user has requested a counting threshold for the CPU cycles event,
then the fixed cycle counter can't be assigned as it lacks threshold
support. Currently, the thresholds will work or not randomly depending
on which counter the event is assigned.

While using thresholds for CPU cycles doesn't make much sense, it can be
useful for testing purposes.

Fixes: 816c2675 ("arm64: perf: Add support for event counting threshold")
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20240626-arm-pmu-3-9-icntr-v2-1-c9784b4f4065@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

drivers/perf/arm_pmuv3.c

@@ -338,6 +338,11 @@ static bool armv8pmu_event_want_user_access(struct perf_event *event)
 	return ATTR_CFG_GET_FLD(&event->attr, rdpmc);
 }
 
+static u32 armv8pmu_event_get_threshold(struct perf_event_attr *attr)
+{
+	return ATTR_CFG_GET_FLD(attr, threshold);
+}
+
 static u8 armv8pmu_event_threshold_control(struct perf_event_attr *attr)
 {
 	u8 th_compare = ATTR_CFG_GET_FLD(attr, threshold_compare);
@@ -941,7 +946,8 @@ static int armv8pmu_get_event_idx(struct pmu_hw_events *cpuc,
 	unsigned long evtype = hwc->config_base & ARMV8_PMU_EVTYPE_EVENT;
 
 	/* Always prefer to place a cycle counter into the cycle counter. */
-	if (evtype == ARMV8_PMUV3_PERFCTR_CPU_CYCLES) {
+	if ((evtype == ARMV8_PMUV3_PERFCTR_CPU_CYCLES) &&
+	    !armv8pmu_event_get_threshold(&event->attr)) {
 		if (!test_and_set_bit(ARMV8_IDX_CYCLE_COUNTER, cpuc->used_mask))
 			return ARMV8_IDX_CYCLE_COUNTER;
 		else if (armv8pmu_event_is_64bit(event) &&
@@ -1033,7 +1039,7 @@ static int armv8pmu_set_event_filter(struct hw_perf_event *event,
 	 * If FEAT_PMUv3_TH isn't implemented, then THWIDTH (threshold_max) will
 	 * be 0 and will also trigger this check, preventing it from being used.
 	 */
-	th = ATTR_CFG_GET_FLD(attr, threshold);
+	th = armv8pmu_event_get_threshold(attr);
 	if (th > threshold_max(cpu_pmu)) {
 		pr_debug("PMU event threshold exceeds max value\n");
 		return -EINVAL;