Wouldn't require different RAM, and wouldn't really require any different register renaming logic. It would however require an extra bit on each register to fla

Wouldn't require different RAM, and wouldn't really require any different register renaming logic. It would however require an extra bit on each register to flag the value as originating from RDRAND, and a different ALU that uses that flag and changes the outcome for certain operations. Obviously, if you store the result of RDRAND to main RAM and read it back in later, this would defeat the system as the flag wouldn't be preserved. But I'm guessing most code won't do that for performance reasons. The simpler option of just making RDRAND predictable is less powerful, because then the operating system can compensate with randomness obtained from elsewhere. The attack above allows the CPU to actually compromise the operating system's own randomness source.