From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Wed, 6 Jul 2016 18:47:00 +0000 (-0400)
Subject: Fix: perf counters: sign-extend pmc register
X-Git-Tag: v2.8.2~16
X-Git-Url: http://git.liburcu.org/?p=lttng-ust.git;a=commitdiff_plain;h=3046b99ebe14547404cb0578c1007aa5eda42472

Fix: perf counters: sign-extend pmc register

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/liblttng-ust/lttng-context-perf-counters.c b/liblttng-ust/lttng-context-perf-counters.c
index 97ddf977..c3728867 100644
--- a/liblttng-ust/lttng-context-perf-counters.c
+++ b/liblttng-ust/lttng-context-perf-counters.c
@@ -110,11 +110,17 @@ uint64_t read_perf_counter(struct perf_event_mmap_page *pc)
 		cmm_barrier();
 
 		idx = pc->index;
-		if (idx)
-			count = pc->offset + rdpmc(idx - 1);
-		else
+		if (idx) {
+			int64_t pmcval;
+
+			pmcval = rdpmc(idx - 1);
+			/* Sign-extend the pmc register result. */
+			pmcval <<= 64 - pc->pmc_width;
+			pmcval >>= 64 - pc->pmc_width;
+			count = pc->offset + pmcval;
+		} else {
 			count = 0;
-
+		}
 		cmm_barrier();
 	} while (CMM_LOAD_SHARED(pc->lock) != seq);