Commit bd9a0c97 authored Jun 19, 2019 by Fenghua Yu Committed by Thomas Gleixner Jun 24, 2019

x86/umwait: Add sysfs interface to control umwait maximum time



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.

The administrator 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. The lower two
bits of the value must be zero.

[ tglx: Simplify the write function. Massage changelog ]

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Cc: "Borislav Petkov" <bp@alien8.de>
Cc: "H Peter Anvin" <hpa@zytor.com>
Cc: "Andy Lutomirski" <luto@kernel.org>
Cc: "Peter Zijlstra" <peterz@infradead.org>
Cc: "Ravi V Shankar" <ravi.v.shankar@intel.com>
Link: https://lkml.kernel.org/r/1560994438-235698-5-git-send-email-fenghua.yu@intel.com

parent ff4b353f

arch/x86/kernel/cpu/umwait.c

+36 −0

Original line number	Diff line number	Diff line
		@@ -131,8 +131,44 @@ 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)
		{
		u32 ctrl = READ_ONCE(umwait_control_cached);

		return sprintf(buf, "%u\n", umwait_ctrl_max_time(ctrl));
		}

		static ssize_t max_time_store(struct device *kobj,
		struct device_attribute *attr,
		const char *buf, size_t count)
		{
		u32 max_time, ctrl;
		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_TIME_MASK)
		return -EINVAL;

		mutex_lock(&umwait_lock);

		ctrl = READ_ONCE(umwait_control_cached);
		if (max_time != umwait_ctrl_max_time(ctrl))
		umwait_update_control(max_time, umwait_ctrl_c02_enabled(ctrl));

		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
		};

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