The plaintext of whatever you're encrypting. Presumably you're going to actually use these debug register AES keys to encrypt or decrypt something more interesting.
The sneaky thing is to do like TreVisor, and I believe the commercial company PrivateCore, to encrypt all memory outside the CPU die (L1/L2, maybe L3?), by pinning the hypervisor and encryption routines to something running inside, and encrypting everything which leaves (and presumably doing some integrity protection). HN user sweis works for PrivateCore; I've talked to them a few times and they seem really interesting, although I think a more conventional HSM makes more sense for some applications, and Intel SGX is going to make the whole thing a lot more interesting in 2-3 years.
