u-boot, following the ePAPR specification, puts secondary cores into a
spinloop at boot, rather than leaving them shut off. It then relies on the host
OS to write the correct values to a special spin table, located in coherent
memory (on newer implementations), or noncoherent memory (older
This supports both implementations of ePAPR, as well as continuing to support
non-ePAPR booting, by first attempting to use the spintable, and falling back to
expecting non-started CPUs.