From fb86534cb16e890fad9e8af43437d488a8b0010b Mon Sep 17 00:00:00 2001
From: "Andrey V. Elsukov" <ae@FreeBSD.org>
Date: Tue, 12 Aug 2014 09:10:13 +0000
Subject: [PATCH] Add sysctl and loader tunable kern.geom.part.mbr.enforce_chs
 that is set by default. It can be used to disable automatic alignment to CHS
 geometry, that GEOM_PART_MBR does.

Reviewed by:	wblock
MFC after:	1 week
---
 sbin/geom/class/part/gpart.8 |  8 +++++++-
 sys/geom/part/g_part_mbr.c   | 10 ++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/sbin/geom/class/part/gpart.8 b/sbin/geom/class/part/gpart.8
index 2345773305c1..236c3fe38d04 100644
--- a/sbin/geom/class/part/gpart.8
+++ b/sbin/geom/class/part/gpart.8
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd August 3, 2014
+.Dd August 12, 2014
 .Dt GPART 8
 .Os
 .Sh NAME
@@ -1183,6 +1183,12 @@ If this variable set to 1 each component of the mirrored volume will be
 present as independent partition.
 .Em NOTE :
 This may break a mirrored volume and lead to data damage.
+.It Va kern.geom.part.mbr.enforce_chs : No 1
+Specify how the Master Boot Record (MBR) module does alignment.
+If this variable is set to a non-zero value, the module will automatically
+recalculate the user-specified offset and size for alignment with the CHS
+geometry.
+Otherwise the values will be left unchanged.
 .El
 .Sh EXIT STATUS
 Exit status is 0 on success, and 1 if the command fails.
diff --git a/sys/geom/part/g_part_mbr.c b/sys/geom/part/g_part_mbr.c
index f2ca6b6443ad..8b32778d23df 100644
--- a/sys/geom/part/g_part_mbr.c
+++ b/sys/geom/part/g_part_mbr.c
@@ -49,6 +49,14 @@ __FBSDID("$FreeBSD$");
 
 FEATURE(geom_part_mbr, "GEOM partitioning class for MBR support");
 
+SYSCTL_DECL(_kern_geom_part);
+static SYSCTL_NODE(_kern_geom_part, OID_AUTO, mbr, CTLFLAG_RW, 0,
+    "GEOM_PART_MBR Master Boot Record");
+
+static u_int enforce_chs = 1;
+SYSCTL_UINT(_kern_geom_part_mbr, OID_AUTO, enforce_chs,
+    CTLFLAG_RWTUN, &enforce_chs, 1, "Enforce alignment to CHS addressing");
+
 #define	MBRSIZE		512
 
 struct g_part_mbr_table {
@@ -200,6 +208,8 @@ mbr_align(struct g_part_table *basetable, uint32_t *start, uint32_t *size)
 {
 	uint32_t sectors;
 
+	if (enforce_chs == 0)
+		return (0);
 	sectors = basetable->gpt_sectors;
 	if (*size < sectors)
 		return (EINVAL);