Otherwise, the guest is running effectively at the same privilege level as the hypervisor (that's useful sometimes, but probably not intended in most applications).
So far seems this is about right:
1. You need platform register access, so seems can't KVM-escape with just this
2. Big question is what about breaking Confidential SEV-SNP guests from the host?
Zen changed DTC (DRAM Controller) to UMC (Unified Memory Controller), UMC is programmed at boot, and one would hope they figured that locking access to it makes sense when they were adding confidential compute support; Not clear though because there is no public documentation on it, so best we can hope for is some statement from AMD/3rd party researcher saying "this won't work on Zen because X/Y/Z"
It's definitely a good research topic because there are a lot of moving pieces and having this kind of primitive might weaken one of them in a useful way, but at least at the top level, you couldn't just swap one guest's DRAM bank with another and get their confidential memory contents back this way.