I'm having a hard time finding out by reading documentation and source 
 code if a system timer IRQ is supposed to be directed at all cores of a 
 processor or if just interrupting one core is enough.

 I know interrupting one core is enough without local timers, because an 
 IPI will be used to interrupt the other. These IPIs are not used with 
 local timers, so does the global timer need to send an IRQ at the same 
 time to both cores in that case?

 Thanks,
 -Chris