[PATCH v5 12/18] x86/split_lock: Enable #AC for split lock by default

[Fenghua Yu]

Split locked access locks bus and degradates overall memory access
performance. When split lock detection feature is enumerated, we want to
enable the feature by default to find any split lock issue and then fix
the issue.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
 arch/x86/kernel/cpu/intel.c | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 889390d51c17..561f7d50246a 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -31,6 +31,11 @@
 #include <asm/apic.h>
 #endif
 
+#define DISABLE_SPLIT_LOCK_DETECT 0
+#define ENABLE_SPLIT_LOCK_DETECT  1
+
+static int split_lock_detect_val;
+
 /*
  * Just in case our CPU detection goes bad, or you have a weird system,
  * allow a way to override the automatic disabling of MPX.
@@ -161,10 +166,35 @@ static bool bad_spectre_microcode(struct cpuinfo_x86 *c)
 	return false;
 }
 
+static u32 new_sp_test_ctl_val(u32 test_ctl_val)
+{
+	/* Change the split lock setting. */
+	if (split_lock_detect_val == DISABLE_SPLIT_LOCK_DETECT)
+		test_ctl_val &= ~TEST_CTL_ENABLE_SPLIT_LOCK_DETECT;
+	else
+		test_ctl_val |= TEST_CTL_ENABLE_SPLIT_LOCK_DETECT;
+
+	return test_ctl_val;
+}
+
+static void init_split_lock_detect(struct cpuinfo_x86 *c)
+{
+	if (cpu_has(c, X86_FEATURE_SPLIT_LOCK_DETECT)) {
+		u32 l, h;
+
+		rdmsr(MSR_TEST_CTL, l, h);
+		l = new_sp_test_ctl_val(l);
+		wrmsr(MSR_TEST_CTL, l, h);
+		pr_info_once("#AC for split lock is enabled\n");
+	}
+}
+
 static void early_init_intel(struct cpuinfo_x86 *c)
 {
 	u64 misc_enable;
 
+	init_split_lock_detect(c);
+
 	/* Unmask CPUID levels if masked: */
 	if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
 		if (msr_clear_bit(MSR_IA32_MISC_ENABLE,
@@ -1048,6 +1078,8 @@ void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c)
 	 */
 	rdmsrl(MSR_IA32_CORE_CAPABILITY, ia32_core_cap);
 
-	if (ia32_core_cap & CORE_CAP_SPLIT_LOCK_DETECT)
+	if (ia32_core_cap & CORE_CAP_SPLIT_LOCK_DETECT) {
 		setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT);
+		split_lock_detect_val = 1;
+	}
 }
-- 
2.19.1