diff --git a/sys/amd64/amd64/amd64_mem.c b/sys/amd64/amd64/amd64_mem.c
index 7704f47dca2c..672e63fbe00a 100644
--- a/sys/amd64/amd64/amd64_mem.c
+++ b/sys/amd64/amd64/amd64_mem.c
@@ -392,6 +392,13 @@ i686_mrsetlow(struct mem_range_softc *sc, struct mem_range_desc *mrd, int *arg)
 	((last_md = i686_mtrrfixsearch(sc, mrd->mr_base + mrd->mr_len - 1)) == NULL))
 	return(EINVAL);
 
+    /* check we aren't doing something risky */
+    if (!(mrd->mr_flags & MDF_FORCE))
+	for (curr_md = first_md; curr_md <= last_md; curr_md++) {
+	    if ((curr_md->mr_flags & MDF_ATTRMASK) == MDF_UNKNOWN)
+		return (EACCES);
+	}
+
     /* set flags, clear set-by-firmware flag */
     for (curr_md = first_md; curr_md <= last_md; curr_md++) {
 	curr_md->mr_flags = mrcopyflags(curr_md->mr_flags & ~MDF_FIRMWARE, mrd->mr_flags);
@@ -431,6 +438,10 @@ i686_mrsetvariable(struct mem_range_softc *sc, struct mem_range_desc *mrd, int *
 		/* whoops, owned by someone */
 		if (curr_md->mr_flags & MDF_BUSY)
 		    return(EBUSY);
+		/* check we aren't doing something risky */
+		if (!(mrd->mr_flags & MDF_FORCE) &&
+		  ((curr_md->mr_flags & MDF_ATTRMASK) == MDF_UNKNOWN))
+		    return (EACCES);
 		/* Ok, just hijack this entry */
 		free_md = curr_md;
 		break;
diff --git a/sys/i386/i386/i686_mem.c b/sys/i386/i386/i686_mem.c
index 7704f47dca2c..672e63fbe00a 100644
--- a/sys/i386/i386/i686_mem.c
+++ b/sys/i386/i386/i686_mem.c
@@ -392,6 +392,13 @@ i686_mrsetlow(struct mem_range_softc *sc, struct mem_range_desc *mrd, int *arg)
 	((last_md = i686_mtrrfixsearch(sc, mrd->mr_base + mrd->mr_len - 1)) == NULL))
 	return(EINVAL);
 
+    /* check we aren't doing something risky */
+    if (!(mrd->mr_flags & MDF_FORCE))
+	for (curr_md = first_md; curr_md <= last_md; curr_md++) {
+	    if ((curr_md->mr_flags & MDF_ATTRMASK) == MDF_UNKNOWN)
+		return (EACCES);
+	}
+
     /* set flags, clear set-by-firmware flag */
     for (curr_md = first_md; curr_md <= last_md; curr_md++) {
 	curr_md->mr_flags = mrcopyflags(curr_md->mr_flags & ~MDF_FIRMWARE, mrd->mr_flags);
@@ -431,6 +438,10 @@ i686_mrsetvariable(struct mem_range_softc *sc, struct mem_range_desc *mrd, int *
 		/* whoops, owned by someone */
 		if (curr_md->mr_flags & MDF_BUSY)
 		    return(EBUSY);
+		/* check we aren't doing something risky */
+		if (!(mrd->mr_flags & MDF_FORCE) &&
+		  ((curr_md->mr_flags & MDF_ATTRMASK) == MDF_UNKNOWN))
+		    return (EACCES);
 		/* Ok, just hijack this entry */
 		free_md = curr_md;
 		break;
diff --git a/sys/i386/i386/k6_mem.c b/sys/i386/i386/k6_mem.c
index 00f9b7ff07ef..7065b98902f4 100644
--- a/sys/i386/i386/k6_mem.c
+++ b/sys/i386/i386/k6_mem.c
@@ -79,7 +79,7 @@ k6_mrmake(struct mem_range_desc *desc, u_int32_t *mtrr) {
 		return EINVAL;
 	if (desc->mr_len < 131072 || !powerof2(desc->mr_len))
 		return EINVAL;
-	if (desc->mr_flags &~ (MDF_WRITECOMBINE|MDF_UNCACHEABLE))
+	if (desc->mr_flags &~ (MDF_WRITECOMBINE|MDF_UNCACHEABLE|MDF_FORCE))
 		return EOPNOTSUPP;
 
 	for (bit = ffs(desc->mr_len >> 17) - 1; bit < 15; bit++)
diff --git a/sys/sys/memrange.h b/sys/sys/memrange.h
index bd18f590b467..558ad31377e3 100644
--- a/sys/sys/memrange.h
+++ b/sys/sys/memrange.h
@@ -20,6 +20,7 @@
 #define MDF_BOGUS		(1<<28)	/* we don't like it */
 #define MDF_FIXACTIVE		(1<<29)	/* can't be turned off */
 #define MDF_BUSY		(1<<30)	/* range is in use */
+#define MDF_FORCE		(1<<31)	/* force risky changes */
 
 struct mem_range_desc
 {
diff --git a/usr.sbin/memcontrol/memcontrol.8 b/usr.sbin/memcontrol/memcontrol.8
index 989eb16f2cf2..f9d24a83ff69 100644
--- a/usr.sbin/memcontrol/memcontrol.8
+++ b/usr.sbin/memcontrol/memcontrol.8
@@ -83,7 +83,8 @@ Length of memory range in bytes, power of 2
 .It Fl o Ar owner
 Text identifier for this setting (7 char max)
 .It Ar attribute
-Attributes applied to this range; one of
+Attributes applied to this range; combinations of
+.Ar force ,
 .Ar uncacheable ,
 .Ar write-combine ,
 .Ar write-through ,
diff --git a/usr.sbin/memcontrol/memcontrol.c b/usr.sbin/memcontrol/memcontrol.c
index 2554723733a9..9eb87197eaaa 100644
--- a/usr.sbin/memcontrol/memcontrol.c
+++ b/usr.sbin/memcontrol/memcontrol.c
@@ -50,6 +50,7 @@ struct
     {"write-through",	MDF_WRITETHROUGH,	MDF_SETTABLE},
     {"write-back",	MDF_WRITEBACK,		MDF_SETTABLE},
     {"write-protect",	MDF_WRITEPROTECT,	MDF_SETTABLE},
+    {"force",		MDF_FORCE,		MDF_SETTABLE},
     {"unknown",		MDF_UNKNOWN,		0},
     {"fixed-base",	MDF_FIXBASE,		0},
     {"fixed-length",	MDF_FIXLEN,		0},