[PATCH 09/15] x86/split_lock: Explicitly enable or disable #AC for split locked accesses

[Fenghua Yu]

By default, we don't set or clear the bit 29 in TEST_CTL MSR 0x33 and
the bit is inherited from BIOS/hardware setting.

The kernel parameter "split_lock_ac=on/off" explicitly sets or clears
the bit during boot time.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt | 11 +++++++++++
 arch/x86/kernel/cpu/split_lock.c                | 16 ++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 11fc28ecdb6d..862758533346 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4030,6 +4030,17 @@
 	spia_pedr=
 	spia_peddr=
 
+	split_lock_ac=	[X86] Enable or disable #AC exception for
+			split locked accesses
+			By default, kernel just inherits setting by
+			hardware and BIOS which may enable or disable
+			the feature. User can explicitly enable or disable
+			the feature by this parameter to override
+			BIOS/hardware setting.
+
+		on	Enable #AC exception for split locked accesses.
+		off	Disable #AC exception for split locked accesses.
+
 	srcutree.counter_wrap_check [KNL]
 			Specifies how frequently to check for
 			grace-period sequence counter wrap for the
diff --git a/arch/x86/kernel/cpu/split_lock.c b/arch/x86/kernel/cpu/split_lock.c
index 5187a9c6cea6..bc66c21b0ca9 100644
--- a/arch/x86/kernel/cpu/split_lock.c
+++ b/arch/x86/kernel/cpu/split_lock.c
@@ -85,6 +85,22 @@ void __init enumerate_split_lock(void)
 	pr_info("#AC exception for split locked accesses is supported\n");
 }
 
+static __init int setup_split_lock_ac(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (!strncmp(str, "on", 2))
+		split_lock_ac = ENABLE_SPLIT_LOCK_AC;
+	else if (!strncmp(str, "off", 3))
+		split_lock_ac = DISABLE_SPLIT_LOCK_AC;
+	else
+		return -EINVAL;
+
+	return 0;
+}
+__setup("split_lock_ac=", setup_split_lock_ac);
+
 static bool _setup_split_lock(int split_lock_ac_val)
 {
 	u32 l, h;
-- 
2.5.0