>The ARM specification mandates that cores in WFI loops should preserve all state. This is not the default mode on Apple Silicon.

I sometimes do wonder at what point do ARM find and test these stuff and ask Apple to follow. Obviously this is not the first time we see such thing happened.

This is allowed as long as you hold the architectural license (which Apple does), and the register is not exposed to userspace (this register is accessible only from kernel)

My understanding is that Apple has an extremely special license with ARM partially because they helped fund ARM when it was spun out of Acorn and they are not required to conform to the specifications like other licensees.