rounding up CPUs on debug exception

[Paul Mackerras]

Geoff Levand writes:

> When kgdb handles a debugging exception (trap or trace for ppc) it 
> tries to stop execution on the other CPUs as soon as possible so 
> that changes in the system state from the time of the exception 
> are minimized.  The i386 code (below) uses the inter-processor 
> interrupt features of the APIC interrupt controller to send a 
> high priority NMI to the other CPUs.  An NMI is needed here, 
> since another CPU may be waiting with interrupts disabled on a 
> spin lock the current cpu holds.

Xmon uses smp_send_debugger_break(MSG_ALL_BUT_SELF) for this.  It
isn't an NMI, just a regular IPI, though.

Paul.