Apply by doing
	cd /sys/arch/sparc/sparc
	patch -p0 < iommureg.patch

Index: iommureg.h
===================================================================
RCS file: /cvs/src/sys/arch/sparc/sparc/iommureg.h,v
retrieving revision 1.4
retrieving revision 1.5
diff -u -r1.4 -r1.5
--- iommureg.h	1998/02/11 01:27:29	1.4
+++ iommureg.h	1998/04/30 06:27:17	1.5
@@ -1,4 +1,4 @@
-/*	$OpenBSD: iommureg.h,v 1.4 1998/02/11 01:27:29 marc Exp $	*/
+/*	$OpenBSD: iommureg.h,v 1.5 1998/04/30 06:27:17 deraadt Exp $	*/
 /*	$NetBSD: iommureg.h,v 1.3 1997/09/14 19:16:04 pk Exp $ */
 
 /*
@@ -61,18 +61,20 @@
 #define IOMMU_FLUSH_MASK	0xfffff000
 
 /*
- * Read something back from the IOMMU control space after writing
- * to a flush register to drain write buffers (?).   This seems to
- * avoid utter lossage on some machines (SS4s & SS5s) where our caller
+ * On SS4/SS5 machines, read something back from the IOMMU control
+ * space after writing to a flush register to drain write buffers (?).
+ * This seems to avoid utter lossage on these machines where our caller
  * would see some of its local (`%lx') registers trashed.
  */
 #define IOMMU_FLUSHPAGE(sc, va)	do {				\
 	(sc)->sc_reg->io_flushpage = (va) & IOMMU_FLUSH_MASK;	\
-	(*(volatile u_int32_t *)&(sc)->sc_reg->io_bar);		\
+	if (cpuinfo.cpu_type == CPUTYP_MS2)			\
+		(*(volatile u_int32_t *)&(sc)->sc_reg->io_bar);	\
 } while (0);
 #define IOMMU_FLUSHALL(sc)	do {				\
 	(sc)->sc_reg->io_flashclear = 0;			\
-	(*(volatile u_int32_t *)&(sc)->sc_reg->io_bar);		\
+	if (cpuinfo.cpu_type == CPUTYP_MS2)			\
+		(*(volatile u_int32_t *)&(sc)->sc_reg->io_bar);	\
 } while (0)
 
 /* to pte.h ? */