Some functionality is accessible via system registers or MMIO e.g. the ETM extension. At the moment, all of memory from 0 to 1 TB is mapped to the primary VM as read-writable device memory but that would likely include the MMIO conduit for features we are trying to block.

We should be more selective about the memory that is exposed to the primary VM and make sure it can't be used to circumvent our protections like this.

See also https://hafnium-review.googlesource.com/c/hafnium/+/7271/.

(Migrated from b/143871895.)