Index: sys/x86/include/clock.h
===================================================================
--- sys/x86/include/clock.h
+++ sys/x86/include/clock.h
@@ -28,6 +28,7 @@
 void	i8254_delay(int);
 void	clock_init(void);
 void	lapic_calibrate(void);
+void	tsc_calibrate(void);
 
 /*
  * Driver to clock driver interface.
Index: sys/x86/isa/clock.c
===================================================================
--- sys/x86/isa/clock.c
+++ sys/x86/isa/clock.c
@@ -413,6 +413,7 @@
 
 	td = curthread;
 
+	tsc_calibrate();
 	lapic_calibrate_timer();
 	cpu_initclocks_bsp();
 	CPU_FOREACH(i) {
@@ -428,6 +429,7 @@
 		sched_unbind(td);
 	thread_unlock(td);
 #else
+	tsc_calibrate();
 	lapic_calibrate_timer();
 	cpu_initclocks_bsp();
 #endif
Index: sys/x86/x86/tsc.c
===================================================================
--- sys/x86/x86/tsc.c
+++ sys/x86/x86/tsc.c
@@ -697,21 +697,24 @@
  * Perform late calibration of the TSC frequency once ACPI-based timecounters
  * are available.
  */
-static void
-tsc_calib(void *arg __unused)
+void
+tsc_calibrate(void)
 {
-	sbintime_t t_start, t_end;
+	struct timecounter *tc;
 	uint64_t freq_khz, tsc_start, tsc_end;
+	u_int t_start, t_end;
 	register_t flags;
 	int cpu;
 
 	if (tsc_disabled)
 		return;
 
+	tc = atomic_load_ptr(&timecounter);
+
 	flags = intr_disable();
 	cpu = curcpu;
 	tsc_start = rdtsc_ordered();
-	t_start = sbinuptime();
+	t_start = tc->tc_get_timecount(tc) & tc->tc_counter_mask;
 	intr_restore(flags);
 
 	DELAY(1000000);
@@ -721,19 +724,23 @@
 
 	flags = intr_disable();
 	tsc_end = rdtsc_ordered();
-	t_end = sbinuptime();
+	t_end = tc->tc_get_timecount(tc) & tc->tc_counter_mask;
 	intr_restore(flags);
 
 	sched_unbind(curthread);
 	thread_unlock(curthread);
 
-	freq_khz = (SBT_1S / 1024) * (tsc_end - tsc_start) / (t_end - t_start);
+	if (t_end <= t_start) {
+		/* Assume that the counter has wrapped around at most once. */
+		t_end += (uint64_t)tc->tc_counter_mask + 1;
+	}
+
+	freq_khz = tc->tc_frequency * (tsc_end - tsc_start) / (t_end - t_start);
 
-	tsc_update_freq(freq_khz * 1024);
+	tsc_update_freq(freq_khz);
 	tc_init(&tsc_timecounter);
 	set_cputicker(rdtsc, tsc_freq, !tsc_is_invariant);
 }
-SYSINIT(tsc_calib, SI_SUB_CLOCKS + 1, SI_ORDER_ANY, tsc_calib, NULL);
 
 void
 resume_TSC(void)