MFC r313447:
Ensure the idle thread's loop services interrupts in a timely way when using the ACPI C1/mwait sleep method. Previously, the mwait instruction would return when an interrupt was pending; however, the idle loop did not actually enable interrupts when this occurred. This led to a situation where the idle loop could quickly spin through the C1/mwait sleep method a number of times when an interrupt was pending. (Eventually, the situation corrected itself when something other than an interrupt triggered the idle loop to either enable interrupts or schedule another thread.)
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