Index: head/sys/kern/subr_unit.c
===================================================================
--- head/sys/kern/subr_unit.c
+++ head/sys/kern/subr_unit.c
@@ -98,6 +98,19 @@
 
 MTX_SYSINIT(unit, &unitmtx, "unit# allocation", MTX_DEF);
 
+#ifdef UNR64_LOCKED
+uint64_t
+alloc_unr64(struct unrhdr64 *unr64)
+{
+	uint64_t item;
+
+	mtx_lock(&unitmtx);
+	item = unr64->counter++;
+	mtx_unlock(&unitmtx);
+	return (item);
+}
+#endif
+
 #else /* ...USERLAND */
 
 #include <bitstring.h>
Index: head/sys/sys/systm.h
===================================================================
--- head/sys/sys/systm.h
+++ head/sys/sys/systm.h
@@ -523,6 +523,32 @@
 int alloc_unrl(struct unrhdr *uh);
 void free_unr(struct unrhdr *uh, u_int item);
 
+#if defined(__mips__) || defined(__powerpc__)
+#define UNR64_LOCKED
+#endif
+
+struct unrhdr64 {
+        uint64_t	counter;
+};
+
+static __inline void
+new_unrhdr64(struct unrhdr64 *unr64, uint64_t low)
+{
+
+	unr64->counter = low;
+}
+
+#ifdef UNR64_LOCKED
+uint64_t alloc_unr64(struct unrhdr64 *);
+#else
+static __inline uint64_t
+alloc_unr64(struct unrhdr64 *unr64)
+{
+
+	return (atomic_fetchadd_64(&unr64->counter, 1));
+}
+#endif
+
 void	intr_prof_stack_use(struct thread *td, struct trapframe *frame);
 
 void counted_warning(unsigned *counter, const char *msg);