The kernel vm region is moved to the last 2GB of the 64-bit address
space. According to the -mcmodel=kernel, the code must be placed in
this range, but probably other addresses can be used for other data
structures, direct memory mapping and so on (as in Linux).
This is just the first step towards being able to use more memory, as
now also user-space has the same 3GB as on a 32-bit kernel, but the
memory structure has not changed much otherwise. The kernel map is not
immediatly following the user map anymore, so the main changes are on
the pmap module.