The Apple M1 SoC has two mechanism for doing IPIs. The first method uses

The Apple M1 SoC has two mechanism for doing IPIs.  The first method uses
the interrupt controller, the second method used implementation-specific
CPU registers.  The M1 Pro/Max SoCs use a modified interrupt controller
that no longer provides the first method.  So switch to the second method
in preparation for adding M1 Pro/Max support to OpenBSD.  As a bonus this
mechanism seems to be faster (which may be why Apple calls these "fast"
IPIs).

ok patrick@