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Subject[PATCH v4 4/5] x86/umwait: Add sysfs interface to control umwait maximum time
Date
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@intel.com>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
---
arch/x86/power/umwait.c | 39 +++++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)

diff --git a/arch/x86/power/umwait.c b/arch/x86/power/umwait.c
index 9c176f3e59b6..7fa381e3fd4e 100644
--- a/arch/x86/power/umwait.c
+++ b/arch/x86/power/umwait.c
@@ -104,8 +104,47 @@ 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_control_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_control_max_time())
+ goto out_unlock;
+
+ umwait_control_cached = max_time | get_umwait_control_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
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