More work on process page tables, including only mapping the last 2 pml4
entries (the highest 1TiB of the address space, ie, kernel space) into a
new table.
Includes the work of actually moving the kernel there, which I had
apparently done in name only previously. Oops.
I forgot to account for tracking page table physical addresses, so
this is a bit of an overhaul. Major changes:
- Refactor bootstrap code into more functions and:
- Only allocate 32 pages of scratch space
- Remap remaining space into top 256GiB, the "page table space"
- Use the page table space to directly offset-map page table pages
from their physical addresses, to avoid tracking overhead.
- Refactor page_block list functions into static functions to better
handle null/empty lists
Move EFI-related code and initial memory manager bootstrap code
to memory_bootstrap.cpp, move memory page structs to their own
memory_page.h/cpp files.
