gistfile1.c

#include <assert.h>
#include <stddef.h>
#include <string.h>
#include "bprintf.h"
#include "mmu.h"

#define IDENT_PDS   64

__attribute__((aligned(4096))) static uintptr_t pml4[512];
__attribute__((aligned(4096))) static uintptr_t pdpt[512];
__attribute__((aligned(4096))) static uintptr_t pd_ident[IDENT_PDS * 512];

void
init_mmu(void)
{
	asm("cli");

	/* Map first 64GiB of physical memory at -512GiB to -448GiB */
	pml4[511] = (uintptr_t)&pdpt | PDE_P | PDE_W | PDE_G;

	for (size_t i = 0; i < IDENT_PDS; i++)
		pdpt[i] = (uintptr_t)&pd_ident[i*512] | PDE_P | PDE_W | PDE_G;

	for (size_t i = 0; i < IDENT_PDS * 512; i++)
		pd_ident[i] = (i * 0x2000000UL) | PDE_P | PDE_W | PDE_PS | PDE_G;

	/* Map the first 1GiB physical to the last 1GiB virtual */
	pdpt[511] = (uintptr_t)&pd_ident[0] | PDE_P | PDE_W;

	/* EFI environment identity maps the lower half, preserve that here. */
	pml4[0] = (uintptr_t)&pdpt | PDE_P | PDE_W | PDE_G;

	asm volatile("movq %0, %%cr3" : : "r"(&pml4) : "memory");
}