On: Verifying ARM processors against the official ARM specification

[AstralStorm]

I wouldn't be so sure to call it intractable. It is pretty hard and time consuming, but not impossible by any stretch of imagination.

Critical parts are hopefully exhaustively tested. Say, memory protection and security.

That said, formal methods are better. ISA-Verifier is a pretty small tool though, nowhere near as strong as Isabelle model used for seL4 for example.