> Memory encryption doesn't prevent you from doing anything with your device, as far as userland is aware they still have access to the full address space with no knowledge encryption is happening
It depends what you mean by "userland". The purpose of SEV is to allow a guest VM (using hardware virtualization) to run without trusting the host, including remote attestation. Traditionally hardware virtualization is used to run a full operating system which was installed at the impetus of the user, but there is no rule that it can only be used for that. If this feature is enabled on desktop parts, it's equally possible for black box DRM software running, say, on a non-virtualized Windows system, to include a small unikernel and automatically set it up to run in SEV mode. The whitepaper proposes that people running VMs in the cloud use remote attestation to upload disk encryption keys such that the VM can only decrypt the disk if it hasn't been tampered with, but the 'cloudiness' could just as well go the other way: cloud DRM servers sending decryption keys, for both video and perhaps the code itself, to enclaves on desktop PCs.
Using SEV alone for DRM would have a significant limitation compared to using the PSP: since all interaction with the outside world is still through the host, it would be hard to prevent the host from grabbing the raw decrypted video data as it leaves. But this still prevents recovering the original bitstream; allows 'perfect' obfuscation of many facets of how exactly the code works; and could probably be used in combination with the PSP in some manner. And in some DRM applications, the ability to grab the output may not matter. Imagine a video game where the bulk of the game was inside an enclave, preventing piracy but also all reverse engineering and modding.
Of course, a video service or video game that only runs on AMD CPUs won't get very popular... but conveniently, Intel is coming out with their own feature, SGX, that provides similar capabilities, though with a different design (it's designed more directly for the DRM use case). One might imagine that eventually most systems will have CPUs that support one or the other.
