Index: head/sys/geom/part/g_part_gpt.c
===================================================================
--- head/sys/geom/part/g_part_gpt.c	(revision 238894)
+++ head/sys/geom/part/g_part_gpt.c	(working copy)
@@ -50,6 +50,15 @@ __FBSDID("$FreeBSD$");
 
 FEATURE(geom_part_gpt, "GEOM partitioning class for GPT partitions support");
 
+SYSCTL_DECL(_kern_geom_part);
+static SYSCTL_NODE(_kern_geom_part, OID_AUTO, gpt, CTLFLAG_RW, 0,
+    "GEOM_PART_GPT GUID Partition Table");
+
+static int gpt_force = 1;
+TUNABLE_INT("kern.geom.part.gpt.force", &gpt_force);
+SYSCTL_UINT(_kern_geom_part_gpt, OID_AUTO, force,
+    CTLFLAG_RW | CTLFLAG_TUN, &gpt_force, 1, "Force GPT detection");
+
 CTASSERT(offsetof(struct gpt_hdr, padding) == 92);
 CTASSERT(sizeof(struct gpt_ent) == 128);
 
@@ -743,6 +752,7 @@ static int
 g_part_gpt_probe(struct g_part_table *table, struct g_consumer *cp)
 {
 	struct g_provider *pp;
+	struct dos_partition *dp;
 	char *buf;
 	int error, res;
 
@@ -774,10 +784,30 @@ g_part_gpt_probe(struct g_part_table *table, struc
 	if (buf == NULL)
 		return (error);
 	res = le16dec(buf + DOSMAGICOFFSET);
+	if (res != DOSMAGIC) {
+		g_free(buf);
+		return (ENXIO);
+	}
+	if (gpt_force == 0) {
+		/* Test Protective MBR for validity */
+		dp = (struct dos_partition *)(buf + DOSPARTOFF);
+		for (res = 0, error = ENXIO; res < NDOSPART; res++) {
+			if (dp[res].dp_flag == 0 &&
+			    dp[res].dp_typ == 0)
+				continue;
+			if (dp[res].dp_flag != 0 &&
+			    dp[res].dp_flag != 0x80)
+				break;
+			if (dp[res].dp_typ == DOSPTYP_PMBR &&
+			    dp[res].dp_start == 1)
+				error = 0;
+		}
+		if (error != 0) {
+			g_free(buf);
+			return (ENXIO);
+		}
+	}
 	g_free(buf);
-	if (res != DOSMAGIC) 
-		return (ENXIO);
-
 	/* Check that there's a primary header. */
 	buf = g_read_data(cp, pp->sectorsize, pp->sectorsize, &error);
 	if (buf == NULL)
Index: head/sbin/geom/class/part/gpart.8
===================================================================
--- head/sbin/geom/class/part/gpart.8	(revision 238894)
+++ head/sbin/geom/class/part/gpart.8	(working copy)
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 19, 2012
+.Dd June 30, 2012
 .Dt GPART 8
 .Os
 .Sh NAME
@@ -957,6 +957,13 @@ disk metadata.
 If some inconsistency is detected, the partition table will be
 rejected with a diagnostic message:
 .Sy "GEOM_PART: Integrity check failed (provider, scheme)" .
+.It Va kern.geom.part.gpt.force : No 1
+This variable controls how the GUID Partition Table (GPT) module does
+validity tests for the Protective MBR.
+The Protective MBR is required by the UEFI specification and in case when
+it is damaged or contains invalid values, GPT will not be detected.
+If this variable set to 1 GPT module ignores some inconsistencies in the
+Protective MBR.
 .It Va kern.geom.part.ldm.debug : No 0
 Debug level of the Logical Disk Manager (LDM) module.
 This can be set to a number between 0 and 2 inclusive.