> I really don't see the point of encrypting memory this way, with what boils down to a global key.

SME was initially developed for game consoles[1], and was designed to protect security keys used for DRM against hardware probing. The feature makes sense in that environment, or any environment where the computer may realistically fall into the hands of an adversary (e.g., a laptop) while still running.

Plain SME on a server doesn't really make sense for DRAM unless your threat model needs to protect against extremely sophisticated attackers (although if there's no real performance hit, you may as well just enable it). However, it would be useful on a system with NVDIMMs.

[1] https://www.crn.com/news/components-peripherals/amd-s-xbox-p...