They have to be same as the maximum addressable bit, i.e. in the case of 48 bit virtual address size the 48th bit.

This is actually kind of a cute way of dividing kernel and userland space as you just set the upper bit to 1 for kernel addresses and 0 for userland.

EDIT: Specifically talking about x86-64 here.

https://github.com/lorenzo-stoakes/linux-mm-notes/blob/maste...