init.c

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#include <core/system.h>
#include <core/string.h>
#include <cpu/cpu.h>
#include <mm/mm.h>
#include <boot/multiboot.h>
#include <boot/boot.h>
 
/* The boot CPU's first page directory, just enough to map the kernel. */
volatile uint32_t _BSP_PD[1024]  __aligned(PAGE_SIZE);
 
#define P   _BV(0)
#define RW  _BV(1)
#define PCD _BV(4)
 
extern char kernel_end;
char scratch[1024 * 1024] __aligned(PAGE_SIZE); /* 1 MiB for the boot page tables */

static inline void enable_paging(uintptr_t page_directory)
{
    write_cr3(page_directory);
    uint32_t cr0 = read_cr0();
 
    /* without WP the kernel writes through read-only pages, e.g. into a
     * copy-on-write page still shared with another process after fork */
    write_cr0(cr0 | CR0_PG | CR0_WP);
}

static void switch_to_higher_half(void)
{
    uint32_t i;
    uintptr_t entries;
 
    /* clear the page directory */
    memset((void *) _BSP_PD, 0, sizeof(_BSP_PD));
 
    /* how many page tables it takes to map the kernel and what follows */
    entries = ARCH_BOOT_MAPPED / TABLE_SIZE;
 
    uint32_t *_BSP_PT = (uint32_t *) scratch;
 
    /* each page mapped to itself */
    for (i = 0; i < entries * 1024; ++i)
        _BSP_PT[i] = (i * PAGE_SIZE) | P | RW;
 
    /* the tables at 0, where the code runs now */
    for (i = 0; i < entries; ++i)
        _BSP_PD[i] = ((uint32_t) _BSP_PT + i * PAGE_SIZE) | P | RW;
 
    /* the same tables in the higher half (3 GiB), where it runs next */
    for (i = 0; i < entries; ++i)
        _BSP_PD[768 + i] = ((uint32_t) _BSP_PT + i * PAGE_SIZE) | P | RW;
 
    /* turn paging on with the boot CPU's page directory */
    enable_paging((uintptr_t) _BSP_PD);
}

void x86_bootstrap()
{
    /* The segments are the boot loader's until x86_cpu_init() loads our
     * GDT: Multiboot leaves them flat, 0 to 4 GiB (x86_64's bootstrap
     * loads its own). First, map the kernel in the higher half. */
    switch_to_higher_half();
 
    /* move the stack pointer to the higher half too */
    extern void early_init_fix_stack(uintptr_t);
    early_init_fix_stack(VMA((uintptr_t) 0));
 
    /* set up the CPU and go on to the kernel */
    extern void x86_cpu_init(void);
    x86_cpu_init();
 
    /* not reached; halt if it ever is */
    for (;;)
        asm volatile ("hlt;");
}