Index: head/sys/boot/kshim/bsd_kernel.c
===================================================================
--- head/sys/boot/kshim/bsd_kernel.c
+++ head/sys/boot/kshim/bsd_kernel.c
@@ -138,6 +138,7 @@
 {
 	int start = ticks;
 	int delta;
+	int time = 0;
 
 	if (cv->sleeping)
 		return (EWOULDBLOCK);	/* not allowed */
@@ -154,6 +155,14 @@
 
 		usb_idle();
 
+		if (++time >= (1000000 / hz)) {
+			time = 0;
+			callout_process(1);
+		}
+
+		/* Sleep for 1 us */
+		delay(1);
+
 		mtx_lock(mtx);
 	}