Index: head/sys/dev/hwpmc/hwpmc_core.c
===================================================================
--- head/sys/dev/hwpmc/hwpmc_core.c
+++ head/sys/dev/hwpmc/hwpmc_core.c
@@ -203,6 +203,10 @@
 static pmc_value_t
 iaf_perfctr_value_to_reload_count(pmc_value_t v)
 {
+
+	/* If the PMC has overflowed, return a reload count of zero. */
+	if ((v & (1ULL << (core_iaf_width - 1))) == 0)
+		return (0);
 	v &= (1ULL << core_iaf_width) - 1;
 	return (1ULL << core_iaf_width) - v;
 }
@@ -1806,6 +1810,10 @@
 static pmc_value_t
 iap_perfctr_value_to_reload_count(pmc_value_t v)
 {
+
+	/* If the PMC has overflowed, return a reload count of zero. */
+	if ((v & (1ULL << (core_iap_width - 1))) == 0)
+		return (0);
 	v &= (1ULL << core_iap_width) - 1;
 	return (1ULL << core_iap_width) - v;
 }
Index: head/sys/dev/hwpmc/hwpmc_mod.c
===================================================================
--- head/sys/dev/hwpmc/hwpmc_mod.c
+++ head/sys/dev/hwpmc/hwpmc_mod.c
@@ -1435,7 +1435,7 @@
 					tmp += pm->pm_sc.pm_reloadcount;
 				mtx_pool_lock_spin(pmc_mtxpool, pm);
 				pp->pp_pmcs[ri].pp_pmcval -= tmp;
-				if ((int64_t) pp->pp_pmcs[ri].pp_pmcval < 0)
+				if ((int64_t) pp->pp_pmcs[ri].pp_pmcval <= 0)
 					pp->pp_pmcs[ri].pp_pmcval +=
 					    pm->pm_sc.pm_reloadcount;
 				mtx_pool_unlock_spin(pmc_mtxpool, pm);