Index: sys/arch/xen/xen/clock.c
===================================================================
RCS file: /cvsroot/src/sys/arch/xen/xen/clock.c,v
retrieving revision 1.64
diff -p -u -r1.64 clock.c
--- sys/arch/xen/xen/clock.c	12 Jun 2016 09:08:09 -0000	1.64
+++ sys/arch/xen/xen/clock.c	29 Oct 2017 16:46:07 -0000
@@ -74,6 +74,8 @@ struct shadow {
 	uint32_t freq_mul;
 	int8_t freq_shift;
 	struct timespec ts;
+	uint64_t last_vcpu_time;
+	bool	last_vcpu_time_set;
 };
 
 /* Protects volatile variables ci_shadow & xen_clock_bias */
@@ -118,22 +120,36 @@ get_time_values_from_xen(struct cpu_info
 	KASSERT(mutex_owned(&tmutex));
 
 	do {
-		shadow->time_version = t->version;
+		if (1 & (shadow->time_version = t->version)) {
+			/* update in progress */
+			SPINLOCK_BACKOFF_HOOK;
+			continue;
+		}
 		xen_rmb();
 		shadow->tsc_stamp = t->tsc_timestamp;
-		shadow->system_time = t->system_time;
+		{
+			uint64_t systime = t->system_time;
+			if (systime < shadow->system_time) {
+				printf("xen systime ran backwards %"PRId64"ns",
+				    shadow->system_time - systime);
+			}
+			shadow->system_time = systime;
+		}
 		shadow->freq_mul = t->tsc_to_system_mul;
 		shadow->freq_shift = t->tsc_shift;
 		xen_rmb();
-	} while ((t->version & 1) || (shadow->time_version != t->version));
+	} while (shadow->time_version != t->version);
 	do {
-		tversion = HYPERVISOR_shared_info->wc_version;
+		if (1 & (tversion = HYPERVISOR_shared_info->wc_version)) {
+			/* update in progress */
+			SPINLOCK_BACKOFF_HOOK;
+			continue;
+		}
 		xen_rmb();
 		shadow->ts.tv_sec = HYPERVISOR_shared_info->wc_sec;
 		shadow->ts.tv_nsec = HYPERVISOR_shared_info->wc_nsec;
 		xen_rmb();
-	} while ((HYPERVISOR_shared_info->wc_version & 1) ||
-	    (tversion != HYPERVISOR_shared_info->wc_version));
+	} while (tversion != HYPERVISOR_shared_info->wc_version);
 }
 
 /*
@@ -192,6 +208,9 @@ get_tsc_offset_ns(struct cpu_info *ci)
 
 	KASSERT(mutex_owned(&tmutex));
 	tsc_delta = cpu_counter() - shadow->tsc_stamp;
+	if ((int64_t)tsc_delta < 0)
+		printf("cpu counter ran behind tsc stamp: %"PRId64"\n",
+		    (int64_t)tsc_delta);
 	offset = scale_delta(tsc_delta, shadow->freq_mul,
 	    shadow->freq_shift);
 
@@ -220,6 +239,14 @@ get_vcpu_time(struct cpu_info *ci)
 
 	} while (!time_values_up_to_date(ci));
 
+	if (shadow->last_vcpu_time_set) {
+		if (stime < shadow->last_vcpu_time)
+			printf("vcpu time ran backwards by %"PRId64"ns\n",
+			    shadow->last_vcpu_time - stime);
+	}
+	shadow->last_vcpu_time = stime;
+	shadow->last_vcpu_time_set = true;
+
 	return stime;
 }