x86: Add NMI callback function.

[stevek]

Summary

Repurpose 'nmi_call_kdb_smp' function to be able to call a callback
function that is meant for safely handling a callback in SMP
environment. The function increments reference on 'nmi_kdb_lock'
atomically for one of the CPU cores. The rest of the CPU cores
execute ia32_pause(). On return from the NMI handler the ref count
on 'nmi_kdb_lock' is dropped and call made to resume the CPU cores.

Obtained from: Juniper Networks, Inc.

[markj]

sys/x86/x86/cpu_machdep.c

884

Doesn't this kind of check now logically belong in the parent? It's a bit odd for the kdb handler to implement this.

897–899

The non-SMP handler should be refactored similarly, no?

[stevek]

sys/x86/x86/cpu_machdep.c

884

Good point. It should be able to move to nmi_handle_intr(), since that's what is called from the trap handler.

897–899

I don't think there's anything that needs to be done for it. It doesn't need to do all the work that the SMP case needs to in a single CPU case (or unicast, not broadcast.)

[stevek]

Updated based on review comments