[PATCH v6 14/20] x86/split_lock: Add a sysfs interface to enable/disable split lock detection during run time

[Fenghua Yu]

The interface /sys/device/system/cpu/split_lock_detect is added
to allow user to control split lock detection and show current split
lock detection setting.

Writing 1 to the file enables split lock detection and writing 0
disables split lock detection. Split lock detection is enabled or
disabled on all CPUs.

Reading the file returns current global split lock detection setting:
0: disabled
1: enabled

Please note the interface only shows global setting. During run time,
split lock detection on one CPU may be disabled if split lock
in kernel code happens on the CPU. The interface doesn't show split
lock detection status on individual CPU. User can run rdmsr on 0x33
to check split lock detection on each CPU.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
 arch/x86/kernel/cpu/intel.c | 66 +++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index ae3e327d5e35..166033fa8423 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1102,3 +1102,69 @@ void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c)
 	if (ia32_core_cap & CORE_CAP_SPLIT_LOCK_DETECT)
 		set_split_lock_detect();
 }
+
+static ssize_t
+split_lock_detect_show(struct device *dev, struct device_attribute *attr,
+		       char *buf)
+{
+	return sprintf(buf, "%u\n", READ_ONCE(split_lock_detect_val));
+}
+
+static ssize_t
+split_lock_detect_store(struct device *dev, struct device_attribute *attr,
+			const char *buf, size_t count)
+{
+	u32 val, l, h;
+	int cpu, ret;
+
+	ret = kstrtou32(buf, 10, &val);
+	if (ret)
+		return ret;
+
+	if (val != DISABLE_SPLIT_LOCK_DETECT && val != ENABLE_SPLIT_LOCK_DETECT)
+		return -EINVAL;
+
+	/*
+	 * Since split lock could be disabled by kernel #AC handler or user
+	 * may directly change bit 29 in MSR_TEST_CTL, split lock setting on
+	 * each CPU may be different from global setting split_lock_detect_val
+	 * by now. Update MSR on each CPU, so all of CPUs will have same split
+	 * lock setting.
+	 */
+	mutex_lock(&split_lock_detect_mutex);
+
+	WRITE_ONCE(split_lock_detect_val, val);
+
+	/*
+	 * Get MSR_TEST_CTL on this CPU, assuming all CPUs have same value
+	 * in the MSR except split lock detection bit (bit 29).
+	 */
+	rdmsr(MSR_TEST_CTL, l, h);
+	l = new_sp_test_ctl_val(l);
+	/* Update the split lock detection setting on all online CPUs. */
+	for_each_online_cpu(cpu)
+		wrmsr_on_cpu(cpu, MSR_TEST_CTL, l, h);
+
+	mutex_unlock(&split_lock_detect_mutex);
+
+	return count;
+}
+
+static DEVICE_ATTR_RW(split_lock_detect);
+
+static int __init split_lock_init(void)
+{
+	int ret;
+
+	if (!boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+		return -ENODEV;
+
+	ret = device_create_file(cpu_subsys.dev_root,
+				 &dev_attr_split_lock_detect);
+	if (ret)
+		return ret;
+
+	return 0;
+}
+
+subsys_initcall(split_lock_init);
-- 
2.19.1