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 <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Ashok Raj <[email protected]>
Reviewed-by: Tony Luck <[email protected]>
Cc: "Borislav Petkov" <[email protected]>
Cc: "H Peter Anvin" <[email protected]>
Cc: "Andy Lutomirski" <[email protected]>
Cc: "Peter Zijlstra" <[email protected]>
Cc: "Ravi V Shankar" <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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