Call critical_enter/critical_exit around (fast) interrupt handlers. All
non-threaded interrupts are fast, and the threaded interrupt scheduler is
itself a fast interrupt.
Assert that an interrupt handler we are about to call is non-zero.
Be paranoid about restoring the users global registers. Do it as the
last thing before switching to alternate globals (when we magically get
our preloaded registers back), and do it with interrupts disabled. Any
kind of kernel trap when the globals are not setup properly is bad news.
Don't save and restore the kernel g6, it invariably points to the current
pcb now.
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