[RFC PATCH 5/8] x86/umwait.c: Add sysfs interface to show tsc_khz

[Fenghua Yu]

User wait process or any other process wants to know tsc frequency to
convert seconds to tsc ticks. Kernel already gets tsc freqency in kernel
internal variable tsc_khz. The sysfs interface /sys/devices/system/cpu/
user_mwait/tsc_khz exposes the internal variable tsc_khz in decimal to
user.

tsc_khz and the interface are available only on CPU that supports
X86_FEATURE_TSC_KNOW_FREQ.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
 arch/x86/power/umwait.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/power/umwait.c b/arch/x86/power/umwait.c
index fd7b18d9ed02..33b3ccb40cb9 100644
--- a/arch/x86/power/umwait.c
+++ b/arch/x86/power/umwait.c
@@ -7,7 +7,8 @@
  */
 /*
  * umwait.c adds control of user wait states that user enters through user wait
- * instructions umwait or tpause.
+ * instructions umwait or tpause. It also dumps tsc_khz to user so user process
+ * can convert seconds to tsc for umwait or other usages.
  */
 #include <linux/cpu.h>
 #include <asm/msr.h>
@@ -49,7 +50,14 @@ static ssize_t umwait_disable_c0_2_store(struct device *dev,
 	return count;
 }
 
+static ssize_t tsc_khz_show(struct device *dev, struct device_attribute *attr,
+			    char *buf)
+{
+	return sprintf(buf, "%d\n", tsc_khz);
+}
+
 static DEVICE_ATTR_RW(umwait_disable_c0_2);
+static DEVICE_ATTR_RO(tsc_khz);
 
 static struct attribute *umwait_attrs[] = {
 	&dev_attr_umwait_disable_c0_2.attr,
@@ -92,6 +100,15 @@ static int __init umwait_init(void)
 	if (ret)
 		return ret;
 
+	/* Only add the tsc_khz interface when the value is known. */
+	if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
+		ret = sysfs_add_file_to_group(&dev->kobj,
+					      &dev_attr_tsc_khz.attr,
+					      umwait_attr_group.name);
+		if (ret)
+			goto out_group;
+	}
+
 	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "umwait/intel:online",
 				umwait_cpu_online, NULL);
 	if (ret < 0)
-- 
2.5.0