MFC r313447:

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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