[PATCH v3 1/1] x86/split_lock: check split lock support on initialization

[Benjamin Lamowski <benjamin.lamowski@kernkonzept.com>]

While the sld setup code is run only if the TEST_CTRL MSR is available,
the current sld initialization code unconditionally resets it even on
systems where this architectural MSR is not available.

This commit introduces a new default sld state sld_unsupported, which is
changed in split_lock_setup() only if sld is available; and checks for
split lock detect support before initializing it.

Fixes: dbaba47085b0c ("x86/split_lock: Rework the initialization flow of split lock detection")
Signed-off-by: Benjamin Lamowski <benjamin.lamowski@kernkonzept.com>
Suggested-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
 arch/x86/kernel/cpu/intel.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 9a26e972cdea..de45ba1089c1 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -34,17 +34,18 @@
 #endif
 
 enum split_lock_detect_state {
-	sld_off = 0,
+	sld_unsupported = 0,
+	sld_off,
 	sld_warn,
 	sld_fatal,
 };
 
 /*
- * Default to sld_off because most systems do not support split lock detection
- * split_lock_setup() will switch this to sld_warn on systems that support
- * split lock detect, unless there is a command line override.
+ * Default to sld_unsupported because most systems do not support split lock
+ * detection. split_lock_setup() will switch this to sld_warn on systems that
+ * support split lock detect, unless there is a command line override.
  */
-static enum split_lock_detect_state sld_state __ro_after_init = sld_off;
+static enum split_lock_detect_state sld_state __ro_after_init = sld_unsupported;
 static u64 msr_test_ctrl_cache __ro_after_init;
 
 /*
@@ -1033,6 +1034,9 @@ static void __init split_lock_setup(void)
 	case sld_fatal:
 		pr_info("sending SIGBUS on user-space split_locks\n");
 		break;
+	case sld_unsupported:
+		/* Can't happen, just to keep the compiler happy */
+		break;
 	}
 
 	rdmsrl(MSR_TEST_CTRL, msr_test_ctrl_cache);
@@ -1063,7 +1067,8 @@ static void sld_update_msr(bool on)
 
 static void split_lock_init(void)
 {
-	split_lock_verify_msr(sld_state != sld_off);
+	if (sld_state != sld_unsupported)
+		split_lock_verify_msr(sld_state != sld_off);
 }
 
 bool handle_user_split_lock(struct pt_regs *regs, long error_code)
-- 
2.25.1