Index: sys/x86/x86/tsc.c
===================================================================
--- sys/x86/x86/tsc.c
+++ sys/x86/x86/tsc.c
@@ -147,6 +147,23 @@
 	tsc_early_calib_exact = 1;
 }
 
+static void
+tsc_freq_xen(void)
+{
+	u_int regs[4];
+
+	/*
+	 * Must run *after* generic tsc_freq_cpuid_vm, so that when Xen is
+	 * emulating Viridian support the Viridian leaf is used instead.
+	 */
+	cpuid_count(0x40000003, 0, regs);
+	tsc_freq = (uint64_t)(regs[2]) * 1000;
+	tsc_early_calib_exact = 1;
+	if (bootverbose)
+		printf("Early TSC frequency %juHz derived from Xen CPUID\n",
+		    (uintmax_t)tsc_freq);
+}
+
 /*
  * Calculate TSC frequency using information from the CPUID leaf 0x15 'Time
  * Stamp Counter and Nominal Core Crystal Clock'.  If leaf 0x15 is not
@@ -318,9 +335,14 @@
 	if (tsc_freq_cpuid_vm())
 		return;
 
-	if (vm_guest == VM_GUEST_VMWARE) {
+	switch (vm_guest) {
+	case VM_GUEST_VMWARE:
 		tsc_freq_vmware();
 		return;
+
+	case VM_GUEST_XEN:
+		tsc_freq_xen();
+		return;
 	}
 
 	if (tsc_freq_cpuid(&tsc_freq)) {