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 83b371c5ba91431f47f44ce6880a4087d2e0244c..61149d061310af0ec8f8f15902b36bff72bb2c15 100644 (file)
--- 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);