The code for handling the messages buffer wasn't up to date in the 2.7 
source. Access to /kern/msgbuf would hang the system and dmesg would
just fail with illegal address. This patch fixes both problems.

Apply by doing:
	cd /usr/src
	patch -p0 < 020_pmax_msgbuf.patch

And then rebuild your kernel.

Index: sys/arch/pmax/include/param.h
===================================================================
RCS file: /cvs/src/sys/arch/pmax/include/param.h,v
retrieving revision 1.8
retrieving revision 1.9
diff -u -r1.8 -r1.9
--- sys/arch/pmax/include/param.h	1997/06/10 14:19:01	1.8
+++ sys/arch/pmax/include/param.h	2000/07/04 05:46:23	1.9
@@ -1,3 +1,4 @@
+/*	$OpenBSD: param.h,v 1.9 2000/07/04 05:46:23 maja Exp $	*/
 /*	$NetBSD: param.h,v 1.18 1997/02/26 01:45:41 jonathan Exp $	*/
 
 /*
@@ -104,6 +105,8 @@
 #define	NMBCLUSTERS	1024		/* map size, max cluster allocation */
 #endif
 #endif
+
+#define MSGBUFSIZE	NBPG
 
 #ifdef _KERNEL
 #ifndef _LOCORE
Index: sys/arch/pmax/pmax/machdep.c
===================================================================
RCS file: /cvs/src/sys/arch/pmax/pmax/machdep.c,v
retrieving revision 1.23
retrieving revision 1.24
diff -u -r1.23 -r1.24
--- sys/arch/pmax/pmax/machdep.c	2000/03/23 09:59:55	1.23
+++ sys/arch/pmax/pmax/machdep.c	2000/07/04 05:46:23	1.24
@@ -1,3 +1,4 @@
+/*	$OpenBSD: machdep.c,v 1.24 2000/07/04 05:46:23 maja Exp $	*/
 /*	$NetBSD: machdep.c,v 1.67 1996/10/23 20:04:40 mhitch Exp $	*/
 
 /*
@@ -147,7 +148,6 @@
 #else
 int	bufpages = 0;
 #endif
-int	msgbufmapped = 0;	/* set when safe to use msgbuf */
 int	maxmem;			/* max memory per process */
 int	physmem;		/* max supported memory, changes to actual */
 int	physmem_boardmax;	/* {model,simm}-specific bound on physmem */
@@ -678,10 +678,10 @@
 	/*
 	 * Initialize error message buffer (at end of core).
 	 */
-	maxmem -= btoc(sizeof (struct msgbuf));
+	maxmem -= btoc(MSGBUFSIZE);
 	msgbufp = (struct msgbuf *)(MIPS_PHYS_TO_KSEG0(maxmem << PGSHIFT));
-	msgbufmapped = 1;
-
+	initmsgbuf((caddr_t)msgbufp, round_page(MSGBUFSIZE));
+	
 	/*
 	 * Allocate space for system data structures.
 	 * The first available kernel virtual address is in "v".
@@ -792,6 +792,11 @@
 	pmapdebug = 0;
 #endif
 
+	for (i = 0; i < btoc(MSGBUFSIZE); i++)
+		pmap_enter(pmap_kernel(), (vm_offset_t)msgbufp,
+		    maxmem + i * NBPG, VM_PROT_READ|VM_PROT_WRITE, TRUE,
+		    VM_PROT_READ|VM_PROT_WRITE);
+
 	/*
 	 * Good {morning,afternoon,evening,night}.
 	 */
@@ -1195,6 +1200,7 @@
 dumpsys()
 {
 	int error;
+	extern int msgbufmapped;
 
 	/* Save registers. */
 	savectx((struct user *)&dumppcb, 0);
Index: sys/arch/pmax/pmax/mem.c
===================================================================
RCS file: /cvs/src/sys/arch/pmax/pmax/mem.c,v
retrieving revision 1.8
retrieving revision 1.9
diff -u -r1.8 -r1.9
--- sys/arch/pmax/pmax/mem.c	1999/11/22 19:22:01	1.8
+++ sys/arch/pmax/pmax/mem.c	2000/07/04 05:46:23	1.9
@@ -1,3 +1,4 @@
+/*	$OpenBSD: mem.c,v 1.9 2000/07/04 05:46:23 maja Exp $	*/
 /*	$NetBSD: mem.c,v 1.7 1995/09/29 21:53:29 jonathan Exp $	*/
 
 /*
@@ -129,8 +130,9 @@
 			c = min(iov->iov_len, MAXPHYS);
 			if (v < MIPS_KSEG0_START)
 				return (EFAULT);
-			if (v + c > MIPS_PHYS_TO_KSEG0(avail_end +
-							sizeof (struct msgbuf)) &&
+			if (v + c > MIPS_PHYS_TO_KSEG0(avail_end) &&
+			   !((caddr_t)v >= (caddr_t)msgbufp &&
+			     (caddr_t)v <  (caddr_t)msgbufp+MSGBUFSIZE) &&
 			    (v < MIPS_KSEG2_START ||
 			    !kernacc((caddr_t)v, c,
 			    uio->uio_rw == UIO_READ ? B_READ : B_WRITE)))