This routine is mainly intended as a tool for applications to
implement LARTs, notably returning the isolation state of a CPU. This
helps in detecting when a user is about to run out-of-band work on a
non-isolated CPU.

Although a dual kernel system performs significantly better than a
native preemption system in this configuration, getting the smallest
jitter and latency figures will require running the out-of-band load
on an isolated core, so that the in-band work does not increase the
rate of cache misses for the out-of-band duties.