powerpc/perf/hv-24x7: Display change in counter values

For 24x7 counters, perf displays the raw value of the 24x7 counter, which
is a monotonically increasing value.

	perf stat -C 0 -e \
		'hv_24x7/HPM_0THRD_NON_IDLE_CCYC__PHYS_CORE,core=1/' \
		sleep 1

 Performance counter stats for 'CPU(s) 0':

     9,105,403,170      hv_24x7/HPM_0THRD_NON_IDLE_CCYC__PHYS_CORE,core=1/

       0.000425751 seconds time elapsed

In the typical usage of 'perf stat' this counter value is not as useful
as the _change_ in the counter value over the duration of the application.

Have h_24x7_event_init() set the event's prev_count to the raw value of
the 24x7 counter at the time of initialization. When the application
terminates, hv_24x7_event_read() will compute the change in value and
report to the perf tool. Similarly, for the transaction interface, clear
the event count to 0 at the beginning of the transaction.

	perf stat -C 0 -e \
		'hv_24x7/HPM_0THRD_NON_IDLE_CCYC__PHYS_CORE,core=1/' \
		sleep 1

 Performance counter stats for 'CPU(s) 0':

           245,758      hv_24x7/HPM_0THRD_NON_IDLE_CCYC__PHYS_CORE,core=1/

       1.006366383 seconds time elapsed
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/perf/hv-24x7.c

@@ -1222,11 +1222,12 @@ static int h_24x7_event_init(struct perf_event *event)
 		return -EACCES;
 	}
 
-	/* see if the event complains */
+	/* Get the initial value of the counter for this event */
 	if (single_24x7_request(event, &ct)) {
 		pr_devel("test hcall failed\n");
 		return -EIO;
 	}
+	(void)local64_xchg(&event->hw.prev_count, ct);
 
 	return 0;
 }
@@ -1289,6 +1290,16 @@ static void h_24x7_event_read(struct perf_event *event)
 			h24x7hw = &get_cpu_var(hv_24x7_hw);
 			h24x7hw->events[i] = event;
 			put_cpu_var(h24x7hw);
+			/*
+			 * Clear the event count so we can compute the _change_
+			 * in the 24x7 raw counter value at the end of the txn.
+			 *
+			 * Note that we could alternatively read the 24x7 value
+			 * now and save its value in event->hw.prev_count. But
+			 * that would require issuing a hcall, which would then
+			 * defeat the purpose of using the txn interface.
+			 */
+			local64_set(&event->count, 0);
 		}
 
 		put_cpu_var(hv_24x7_reqb);