Intel's "Direct Data I/O" feature introduced in 2011 has been shown to allow for capturing keystrokes in "keyboard interactive" SSH sessions (archived). Keyboard interactive logins have been a feature of SSH best disabled for some time, but Intel's DDIO backdoor is architected in a way that allows still further shennanigans. The DDIO "optimization" allows peripherals to go further than direct memory address and skip to directly interacting with the CPU's on die cache. Expect the continued documentation of Intel backdoors marketed as optimizations to continue.