perf, x86: Provide a PEBS capable cycle event

Signed-off-by: Peter Zijlstra <[e-mail]>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/cpu/perf_event_intel.c

···
 	if (ret)
 		return ret;
 
+	if (event->attr.precise_ip &&
+	    (event->hw.config & X86_RAW_EVENT_MASK) == 0x003c) {
+		/*
+		 * Use an alternative encoding for CPU_CLK_UNHALTED.THREAD_P
+		 * (0x003c) so that we can use it with PEBS.
+		 *
+		 * The regular CPU_CLK_UNHALTED.THREAD_P event (0x003c) isn't
+		 * PEBS capable. However we can use INST_RETIRED.ANY_P
+		 * (0x00c0), which is a PEBS capable event, to get the same
+		 * count.
+		 *
+		 * INST_RETIRED.ANY_P counts the number of cycles that retires
+		 * CNTMASK instructions. By setting CNTMASK to a value (16)
+		 * larger than the maximum number of instructions that can be
+		 * retired per cycle (4) and then inverting the condition, we
+		 * count all cycles that retire 16 or less instructions, which
+		 * is every cycle.
+		 *
+		 * Thereby we gain a PEBS capable cycle counter.
+		 */
+		u64 alt_config = 0x108000c0; /* INST_RETIRED.TOTAL_CYCLES */
+
+		alt_config |= (event->hw.config & ~X86_RAW_EVENT_MASK);
+		event->hw.config = alt_config;
+	}
+
 	if (event->attr.type != PERF_TYPE_RAW)
 		return 0;