The XIVE (External Interrupt Virtualization Engine) is a new interrupt
controller present in IBM's POWER9 processor. It's a very powerful,
very complex device using queues and shared memory to improve interrupt
dispatch performance in a virtualized environment.
This yields a ~10% performance improvment over the XICS emulation mode,
measured in both buildworld, and 'dd' from nvme to /dev/null.
Currently, this only supports native access.
MFC after: 1 month
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