arm64: perf: Allow more than one cycle counter to be used

Currently:
$ perf stat -e cycles:u -e cycles:k  true

 Performance counter stats for 'true':

          2,24,699      cycles:u
     <not counted>      cycles:k	(0.00%)

       0.000788087 seconds time elapsed

We can not count more than one cycle counter in one instance,because we
allow to map cycle counter into PMCCNTR_EL0 only. However, if I did not
miss anything then specification do not prohibit to use PMEVCNTR<n>_EL0
for cycle count as well.

Modify the code so that it still prefers to use PMCCNTR_EL0 for cycle
counter, however allow to use PMEVCNTR<n>_EL0 if PMCCNTR_EL0 is already
in use.

After this patch:

$ perf stat -e cycles:u -e cycles:k   true

 Performance counter stats for 'true':

          2,17,310      cycles:u
          7,40,009      cycles:k

       0.000764149 seconds time elapsed
Signed-off-by: Pratyush Anand <panand@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/perf_event.c

@@ -846,17 +846,14 @@ static int armv8pmu_get_event_idx(struct pmu_hw_events *cpuc,
 	struct hw_perf_event *hwc = &event->hw;
 	unsigned long evtype = hwc->config_base & ARMV8_PMU_EVTYPE_EVENT;
 
-	/* Always place a cycle counter into the cycle counter. */
+	/* Always prefer to place a cycle counter into the cycle counter. */
 	if (evtype == ARMV8_PMUV3_PERFCTR_CPU_CYCLES) {
-		if (test_and_set_bit(ARMV8_IDX_CYCLE_COUNTER, cpuc->used_mask))
-			return -EAGAIN;
-
+		if (!test_and_set_bit(ARMV8_IDX_CYCLE_COUNTER, cpuc->used_mask))
 			return ARMV8_IDX_CYCLE_COUNTER;
 	}
 
 	/*
-	 * For anything other than a cycle counter, try and use
-	 * the events counters
+	 * Otherwise use events counters
 	 */
 	for (idx = ARMV8_IDX_COUNTER0; idx < cpu_pmu->num_events; ++idx) {
 		if (!test_and_set_bit(idx, cpuc->used_mask))