[PATCH v8 15/15] x86/split_lock: Add a sysfs interface to enable/disable split lock detection during run time

[Fenghua Yu]

To workaround or debug a split lock issue, the administrator may need to
disable or enable split lock detection during run time without rebooting
the system.

The interface /sys/device/system/cpu/split_lock_detect is added to allow
the administrator to disable or enable split lock detection and show
current split lock detection setting.

Writing [yY1] or [oO][nN] to the file enables split lock detection and
writing [nN0] or [oO][fF] 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

Add an ABI document entry for /sys/devices/system/cpu/split_lock_detect.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
Not sure if the justification for the sysfs knob is valid. If not, this
patch could be removed from this patch set.

 .../ABI/testing/sysfs-devices-system-cpu      | 22 ++++++++
 arch/x86/kernel/cpu/intel.c                   | 52 ++++++++++++++++++-
 2 files changed, 72 insertions(+), 2 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 9605dbd4b5b5..aad7b1698065 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -67,6 +67,28 @@ Description:	Discover NUMA node a CPU belongs to
 		/sys/devices/system/cpu/cpu42/node2 -> ../../node/node2
 
 
+What:		/sys/devices/system/cpu/split_lock_detect
+Date:		March 2019
+Contact:	Linux kernel mailing list <linux-kernel@xxxxxxxxxxxxxxx>
+Description:	(RW) Control split lock detection on Intel Tremont and
+		future CPUs
+
+		Reads return split lock detection status:
+			0: disabled
+			1: enabled
+
+		Writes enable or disable split lock detection:
+			The first character is one of 'Nn0' or [oO][fF] for off
+			disables the feature.
+			The first character is one of 'Yy1' or [oO][nN] for on
+			enables the feature.
+
+		Please note the interface only shows or controls global setting.
+		During run time, split lock detection on one CPU may be
+		disabled if split lock operation in kernel code happens on
+		the CPU. The interface doesn't show or control split lock
+		detection on individual CPU.
+
 What:		/sys/devices/system/cpu/cpu#/topology/core_id
 		/sys/devices/system/cpu/cpu#/topology/core_siblings
 		/sys/devices/system/cpu/cpu#/topology/core_siblings_list
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 959ebf25beda..f257d1e92706 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -35,6 +35,7 @@
 DEFINE_PER_CPU(u64, msr_test_ctl_cache);
 EXPORT_PER_CPU_SYMBOL_GPL(msr_test_ctl_cache);
 
+static DEFINE_MUTEX(split_lock_detect_mutex);
 static bool split_lock_detect_enable;
 
 /*
@@ -660,7 +661,7 @@ static void init_intel_misc_features(struct cpuinfo_x86 *c)
 	wrmsrl(MSR_MISC_FEATURES_ENABLES, msr);
 }
 
-static void split_lock_update_msr(void)
+static void split_lock_update_msr(void *__unused)
 {
 	if (split_lock_detect_enable) {
 		/* Enable split lock detection */
@@ -682,7 +683,7 @@ static void init_split_lock_detect(struct cpuinfo_x86 *c)
 		rdmsrl(MSR_TEST_CTL, test_ctl_val);
 		this_cpu_write(msr_test_ctl_cache, test_ctl_val);
 
-		split_lock_update_msr();
+		split_lock_update_msr(NULL);
 	}
 }
 
@@ -1114,3 +1115,50 @@ void handle_split_lock_kernel_mode(void)
 	this_cpu_and(msr_test_ctl_cache, ~TEST_CTL_SPLIT_LOCK_DETECT);
 	WARN_ONCE(1, "split lock operation detected\n");
 }
+
+static ssize_t
+split_lock_detect_show(struct device *dev, struct device_attribute *attr,
+		       char *buf)
+{
+	return sprintf(buf, "%u\n", split_lock_detect_enable);
+}
+
+static ssize_t
+split_lock_detect_store(struct device *dev, struct device_attribute *attr,
+			const char *buf, size_t count)
+{
+	bool val;
+	int ret;
+
+	ret = strtobool(buf, &val);
+	if (ret)
+		return ret;
+
+	mutex_lock(&split_lock_detect_mutex);
+
+	split_lock_detect_enable = val;
+
+	/* Update the split lock detection setting in MSR on all online CPUs. */
+	on_each_cpu(split_lock_update_msr, NULL, 1);
+
+	if (split_lock_detect_enable)
+		pr_info("enabled\n");
+	else
+		pr_info("disabled\n");
+
+	mutex_unlock(&split_lock_detect_mutex);
+
+	return count;
+}
+
+static DEVICE_ATTR_RW(split_lock_detect);
+
+static int __init split_lock_init(void)
+{
+	if (!boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+		return -ENODEV;
+
+	return device_create_file(cpu_subsys.dev_root,
+				  &dev_attr_split_lock_detect);
+}
+subsys_initcall(split_lock_init);
-- 
2.19.1