Index: head/sys/dev/hyperv/include/hyperv.h
===================================================================
--- head/sys/dev/hyperv/include/hyperv.h
+++ head/sys/dev/hyperv/include/hyperv.h
@@ -36,6 +36,23 @@
 #include <vm/vm.h>
 #include <vm/pmap.h>
 
+#define MSR_HV_TIME_REF_COUNT		0x40000020
+
+#define CPUID_HV_MSR_TIME_REFCNT	0x0002	/* MSR_HV_TIME_REF_COUNT */
+#define CPUID_HV_MSR_SYNIC		0x0004	/* MSRs for SynIC */
+#define CPUID_HV_MSR_SYNTIMER		0x0008	/* MSRs for SynTimer */
+#define CPUID_HV_MSR_APIC		0x0010	/* MSR_HV_{EOI,ICR,TPR} */
+#define CPUID_HV_MSR_HYPERCALL		0x0020	/* MSR_HV_GUEST_OS_ID
+						 * MSR_HV_HYPERCALL */
+#define CPUID_HV_MSR_VP_INDEX		0x0040	/* MSR_HV_VP_INDEX */
+#define CPUID_HV_MSR_GUEST_IDLE		0x0400	/* MSR_HV_GUEST_IDLE */
+
+#ifndef NANOSEC
+#define NANOSEC				1000000000ULL
+#endif
+#define HYPERV_TIMER_NS_FACTOR		100ULL
+#define HYPERV_TIMER_FREQ		(NANOSEC / HYPERV_TIMER_NS_FACTOR)
+
 struct hyperv_guid {
 	uint8_t		hv_guid[16];
 } __packed;
@@ -44,4 +61,6 @@
 
 int		hyperv_guid2str(const struct hyperv_guid *, char *, size_t);
 
+extern u_int	hyperv_features;	/* CPUID_HV_MSR_ */
+
 #endif  /* _HYPERV_H_ */
Index: head/sys/dev/hyperv/utilities/hv_timesync.c
===================================================================
--- head/sys/dev/hyperv/utilities/hv_timesync.c
+++ head/sys/dev/hyperv/utilities/hv_timesync.c
@@ -46,10 +46,14 @@
 #define VMBUS_TIMESYNC_FWVER		\
 	VMBUS_IC_VERSION(VMBUS_TIMESYNC_FWVER_MAJOR, 0)
 
-#define VMBUS_TIMESYNC_MSGVER_MAJOR	3
+#define VMBUS_TIMESYNC_MSGVER_MAJOR	4
 #define VMBUS_TIMESYNC_MSGVER		\
 	VMBUS_IC_VERSION(VMBUS_TIMESYNC_MSGVER_MAJOR, 0)
 
+#define VMBUS_TIMESYNC_DORTT(sc)	\
+	((sc)->ic_msgver >= VMBUS_IC_VERSION(4, 0) && \
+	 (hyperv_features & CPUID_HV_MSR_TIME_REFCNT))
+
 static const struct vmbus_ic_desc vmbus_timesync_descs[] = {
 	{
 		.ic_guid = { .hv_guid = {
@@ -81,12 +85,16 @@
     &vmbus_ts_sample_verbose, 0, "Increase sample request verbosity.");
 
 static void
-vmbus_timesync(struct hv_util_sc *sc, uint64_t hvtime, uint8_t tsflags)
+vmbus_timesync(struct hv_util_sc *sc, uint64_t hvtime, uint64_t sent_tc,
+    uint8_t tsflags)
 {
 	struct timespec vm_ts;
-	uint64_t hv_ns, vm_ns;
+	uint64_t hv_ns, vm_ns, rtt = 0;
+
+	if (VMBUS_TIMESYNC_DORTT(sc))
+		rtt = rdmsr(MSR_HV_TIME_REF_COUNT) - sent_tc;
 
-	hv_ns = (hvtime - VMBUS_ICMSG_TS_BASE) * VMBUS_ICMSG_TS_FACTOR;
+	hv_ns = (hvtime - VMBUS_ICMSG_TS_BASE + rtt) * HYPERV_TIMER_NS_FACTOR;
 	nanotime(&vm_ts);
 	vm_ns = (vm_ts.tv_sec * NANOSEC) + vm_ts.tv_nsec;
 
@@ -174,6 +182,8 @@
 		    VMBUS_TIMESYNC_FWVER, VMBUS_TIMESYNC_MSGVER);
 		if (error)
 			return;
+		if (VMBUS_TIMESYNC_DORTT(sc))
+			device_printf(sc->ic_dev, "RTT\n");
 		break;
 
 	case VMBUS_ICMSG_TYPE_TIMESYNC:
@@ -183,7 +193,8 @@
 			return;
 		}
 		msg = data;
-		vmbus_timesync(sc, msg->ic_hvtime, msg->ic_tsflags);
+		vmbus_timesync(sc, msg->ic_hvtime, msg->ic_sent_tc,
+		    msg->ic_tsflags);
 		break;
 
 	default:
Index: head/sys/dev/hyperv/utilities/vmbus_icreg.h
===================================================================
--- head/sys/dev/hyperv/utilities/vmbus_icreg.h
+++ head/sys/dev/hyperv/utilities/vmbus_icreg.h
@@ -114,18 +114,13 @@
 	struct vmbus_icmsg_hdr	ic_hdr;
 	uint64_t		ic_hvtime;
 	uint64_t		ic_vmtime;
-	uint64_t		ic_rtt;
+	uint64_t		ic_sent_tc;
 	uint8_t			ic_tsflags;	/* VMBUS_ICMSG_TS_FLAG_ */
 } __packed;
 
 #define VMBUS_ICMSG_TS_FLAG_SYNC	0x01
 #define VMBUS_ICMSG_TS_FLAG_SAMPLE	0x02
 
-/* XXX consolidate w/ hyperv */
 #define VMBUS_ICMSG_TS_BASE		116444736000000000ULL
-#define VMBUS_ICMSG_TS_FACTOR		100ULL
-#ifndef NANOSEC
-#define NANOSEC				1000000000ULL
-#endif
 
 #endif	/* !_VMBUS_ICREG_H_ */
Index: head/sys/dev/hyperv/vmbus/hyperv_reg.h
===================================================================
--- head/sys/dev/hyperv/vmbus/hyperv_reg.h
+++ head/sys/dev/hyperv/vmbus/hyperv_reg.h
@@ -57,8 +57,6 @@
 
 #define MSR_HV_VP_INDEX			0x40000002
 
-#define MSR_HV_TIME_REF_COUNT		0x40000020
-
 #define MSR_HV_SCONTROL			0x40000080
 #define MSR_HV_SCTRL_ENABLE		0x0001ULL
 #define MSR_HV_SCTRL_RSVD_MASK		0xfffffffffffffffeULL
@@ -106,15 +104,7 @@
 #define CPUID_LEAF_HV_IDENTITY		0x40000002
 
 #define CPUID_LEAF_HV_FEATURES		0x40000003
-/* EAX: features */
-#define CPUID_HV_MSR_TIME_REFCNT	0x0002	/* MSR_HV_TIME_REF_COUNT */
-#define CPUID_HV_MSR_SYNIC		0x0004	/* MSRs for SynIC */
-#define CPUID_HV_MSR_SYNTIMER		0x0008	/* MSRs for SynTimer */
-#define CPUID_HV_MSR_APIC		0x0010	/* MSR_HV_{EOI,ICR,TPR} */
-#define CPUID_HV_MSR_HYPERCALL		0x0020	/* MSR_HV_GUEST_OS_ID
-						 * MSR_HV_HYPERCALL */
-#define CPUID_HV_MSR_VP_INDEX		0x0040	/* MSR_HV_VP_INDEX */
-#define CPUID_HV_MSR_GUEST_IDLE		0x0400	/* MSR_HV_GUEST_IDLE */
+/* EAX: features include/hyperv.h CPUID_HV_MSR */
 /* ECX: power management features */
 #define CPUPM_HV_CSTATE_MASK		0x000f	/* deepest C-state */
 #define CPUPM_HV_C3_HPET		0x0010	/* C3 requires HPET */
Index: head/sys/dev/hyperv/vmbus/hyperv_var.h
===================================================================
--- head/sys/dev/hyperv/vmbus/hyperv_var.h
+++ head/sys/dev/hyperv/vmbus/hyperv_var.h
@@ -29,13 +29,6 @@
 #ifndef _HYPERV_VAR_H_
 #define _HYPERV_VAR_H_
 
-#ifndef NANOSEC
-#define NANOSEC			1000000000ULL
-#endif
-#define HYPERV_TIMER_NS_FACTOR	100ULL
-#define HYPERV_TIMER_FREQ	(NANOSEC / HYPERV_TIMER_NS_FACTOR)
-
-extern u_int	hyperv_features;
 extern u_int	hyperv_recommends;
 
 uint64_t	hypercall_post_message(bus_addr_t msg_paddr);
Index: head/sys/dev/hyperv/vmbus/vmbus_et.c
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus_et.c
+++ head/sys/dev/hyperv/vmbus/vmbus_et.c
@@ -36,6 +36,7 @@
 #include <sys/systm.h>
 #include <sys/timeet.h>
 
+#include <dev/hyperv/include/hyperv.h>
 #include <dev/hyperv/vmbus/hyperv_reg.h>
 #include <dev/hyperv/vmbus/hyperv_var.h>
 #include <dev/hyperv/vmbus/vmbus_var.h>