Index: sys/_endian.h
===================================================================
RCS file: /cvs/src/sys/sys/_endian.h,v
retrieving revision 1.2
diff -u -p -r1.2 _endian.h
--- sys/_endian.h	21 Apr 2017 19:04:22 -0000	1.2
+++ sys/_endian.h	5 Jun 2017 01:11:00 -0000
@@ -104,6 +104,21 @@
  * where we can take advantage of the CSE phase much better by using the
  * generic version.
  */
+
+/*
+ * Unless provided by the machine, the kernel will fall back to function
+ * calls for "MD" swap operations.
+ */
+#ifdef _KERNEL
+#ifndef __HAVE_MD_SWAP
+__uint16_t __swap16md(__uint16_t);
+__uint32_t __swap32md(__uint32_t);
+__uint64_t __swap64md(__uint64_t);
+#define __HAVE_EMUL_MD_SWAP /* for sys/kern/kern_subr.c */
+#define __HAVE_MD_SWAP
+#endif /* !__HAVE_MD_SWAP */
+#endif /* _KERNEL */
+
 #ifdef __HAVE_MD_SWAP
 #if __GNUC__
 
Index: kern/kern_subr.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_subr.c,v
retrieving revision 1.49
diff -u -p -r1.49 kern_subr.c
--- kern/kern_subr.c	14 Feb 2017 10:31:15 -0000	1.49
+++ kern/kern_subr.c	5 Jun 2017 01:11:00 -0000
@@ -44,6 +44,7 @@
 #include <sys/malloc.h>
 #include <sys/queue.h>
 #include <sys/resourcevar.h>
+#include <sys/endian.h>
 
 int
 uiomove(void *cp, size_t n, struct uio *uio)
@@ -257,3 +258,23 @@ dohooks(struct hook_desc_head *head, int
 		}
 	}
 }
+
+#ifdef __HAVE_EMUL_MD_SWAP
+__uint16_t
+__swap16md(__uint16_t x)
+{
+	return (__swap16gen(x));
+}
+
+__uint32_t
+__swap32md(__uint32_t x)
+{
+	return (__swap32gen(x));
+}
+
+__uint64_t
+__swap64md(__uint64_t x)
+{
+	return (__swap64gen(x));
+}
+#endif