Kernel loading into higher half memory - #2
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Restructured the linker to have a new boot section and start using virtual memory for the main sections of the kernel, while boot section uses physical memory. Idea is to have a baked in bootstrap page embedded into the kernel binary, which will be used to initially start virtual memory.
The kernels entry has been refactored to now switch the kernel to high half virtual address, and sets up a single gigapage leaf that adds the mapping. And the identity mapping and the dtb (if it happens to lie in a different gigapage)
The reason for adding the extra argument for main is that high-half compiled code cannot conveniently form the address of a linker symbol whose value lives extremely far away in the low physical address range. Updated and rewrote relevant function and docs for getting the kernels physical memory range.
Made documentation more consistent and reflect the new changes added which i missed earlier.
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Made the kernel now boot into main such that it already is in higher half virtual memory.
Achieved my reconfiguing the linker (and readjusting the compile "recipie")
Having the produced binary be such that there is "boot" part that is configured to be loaded and run on physical memory, set by the nix configuration. It's responsbile for setting a "gigapage" (a Sv39 root leaf mappings). That maps higher half address and identity maps the physical addresses intially to survive the switch.
Once mapping is active it jumps to that used to be the old
_startbut in higher address space and only then jumps tomainwhich enter fully in virtual space. Point is this internal root page is baked into the binary to avoid the problem reagrding needing fdt to discover memory.