[PATCH v5 4/5] x86/umwait: Add sysfs interface to control umwait maximum time

[Fenghua Yu]

IA32_UMWAIT_CONTROL[31:2] determines the maximum time in TSC-quanta
that processor can stay in C0.1 or C0.2. A zero value means no maximum
time.

Each instruction sets its own deadline in the instruction's implicit
input EDX:EAX value. The instruction wakes up if the time-stamp counter
reaches or exceeds the specified deadline, or the umwait maximum time
expires, or a store happens in the monitored address range in umwait.

Users can write an unsigned 32-bit number to
/sys/devices/system/cpu/umwait_control/max_time to change the default
value. Note that a value of zero means there is no limit. Low order
two bits must be zero.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Reviewed-by: Ashok Raj <ashok.raj@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
---
 arch/x86/kernel/cpu/umwait.c | 40 ++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/arch/x86/kernel/cpu/umwait.c b/arch/x86/kernel/cpu/umwait.c
index 3bd6d37a7b2c..4b2aff7b2d4d 100644
--- a/arch/x86/kernel/cpu/umwait.c
+++ b/arch/x86/kernel/cpu/umwait.c
@@ -126,8 +126,48 @@ static ssize_t enable_c02_store(struct device *dev,
 }
 static DEVICE_ATTR_RW(enable_c02);
 
+static ssize_t
+max_time_show(struct device *kobj, struct device_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%u\n", get_umwait_ctrl_max_time());
+}
+
+static ssize_t max_time_store(struct device *kobj,
+			      struct device_attribute *attr,
+			      const char *buf, size_t count)
+{
+	u32 max_time;
+	int ret;
+
+	ret = kstrtou32(buf, 0, &max_time);
+	if (ret)
+		return ret;
+
+	/* bits[1:0] must be zero */
+	if (max_time & ~MSR_IA32_UMWAIT_CONTROL_MAX_TIME)
+		return -EINVAL;
+
+	mutex_lock(&umwait_lock);
+
+	if (max_time == get_umwait_ctrl_max_time())
+		goto out_unlock;
+
+	WRITE_ONCE(umwait_control_cached,
+		   UMWAIT_CTRL_VAL(max_time, get_umwait_ctrl_c02()));
+
+	/* Update umwait max time on all CPUs */
+	on_each_cpu(umwait_control_msr_update, NULL, 1);
+
+out_unlock:
+	mutex_unlock(&umwait_lock);
+
+	return count;
+}
+static DEVICE_ATTR_RW(max_time);
+
 static struct attribute *umwait_attrs[] = {
 	&dev_attr_enable_c02.attr,
+	&dev_attr_max_time.attr,
 	NULL
 };
 
-- 
2.19.1