calcite/kernel/src/paging.c

/* Calcite, kernel/src/paging.c
 * Copyright 2025 Benji Dial
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or (at
 * your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program. If not, see <https://www.gnu.org/licenses/>.
 */

#include <paging.h>

#define MAX_PHYSICAL_GB 64ULL

//this is a simple bitmap. physical_map[i / 8] & (1 << (i % 8)) is nonzero if
//and only if the physical page with base i * 4096 is free.
static uint8_t physical_map[MAX_PHYSICAL_GB << 15];

//kernel virtual memory is the top gigabyte, i.e. one "page directory" worth
//of pages. in the future i might make it more reactive, and allow new page
//directories to be allocated as needed, but for now there is just the one
//page directory, and all of the page tables in that directory are statically
//allocated. i do not like the intel/amd names for the paging structures, so
//i will use these names throughout the rest of this file:
//  p4 = "page map level 4"
//  p3 = "page directory pointer table"
//  p2 = "page directory"
//  p1 = "page table"

//referenced from paging.asm
uint64_t kernel_p4_physical_address;

alignas(4096) static uint64_t kernel_p4[512];
alignas(4096) static uint64_t kernel_p3[512];
alignas(4096) static uint64_t kernel_p2[512];
alignas(4096) static uint64_t kernel_p1s[512 * 512];

//referenced from paging.asm. at the end of init, this will either be marked
//free or reused as an interrupt / syscall stack. if the length of this is
//changed, it also needs to be changed in paging.asm. there is no guard page,
//but i don't expect init to need much stack.
alignas(4096) uint64_t init_stack[16384];

void init_paging(uint64_t kernel_physical_base, void *kernel_virtual_base) {

  uint64_t kernel_load_offset =
    (uint64_t)kernel_virtual_base - kernel_physical_base;

  kernel_p4_physical_address = (uint64_t)kernel_p4 - kernel_load_offset;

  kernel_p4[511] = ((uint64_t)kernel_p3 - kernel_load_offset) | 0x3;
  for (int i = 0; i < 511; ++i)
    kernel_p4[i] = 0;

  kernel_p3[511] = ((uint64_t)kernel_p2 - kernel_load_offset) | 0x3;
  for (int i = 0; i < 511; ++i)
    kernel_p3[i] = 0;

  for (int i = 0; i < 512; ++i)
    kernel_p2[i] =
      ((uint64_t)kernel_p1s - kernel_load_offset + i * 4096) | 0x03;

  for (int i = 0; i < 512 * 512; ++i)
    kernel_p1s[i] = 0;

}

void mark_physical_memory_free(uint64_t base, uint64_t length) {

  if (base + length > MAX_PHYSICAL_GB << 30)
    length = (MAX_PHYSICAL_GB << 30) - base;

  while (length && (base & (0x7 << 12))) {
    physical_map[base >> 15] |= 1 << ((base >> 12) % 8);
    base += 4096;
    length -= 4096;
  }

  while (length >= 4096 * 8) {
    physical_map[base >> 15] = 0xff;
    base += 4096 * 8;
    length -= 4096 * 8;
  }

  while (length) {
    physical_map[base >> 15] |= 1 << ((base >> 12) % 8);
    base += 4096;
    length -= 4096;
  }

}

void map_in_kernel_page_table(
    uint64_t physical_base, void *virtual_base,
    int writable, int executable) {

  uint64_t virtual_base_u64 = (uint64_t)virtual_base;

  //should probably die in this case
  if (virtual_base_u64 < 0xffffffffc0000000)
    return;

  uint64_t p1s_index = (virtual_base_u64 - 0xffffffffc0000000) >> 12;

  //should probably die in this case too
  if (kernel_p1s[p1s_index] != 0)
    return;

  kernel_p1s[p1s_index] =
    physical_base | (writable ? 0x3 : 0x1) |
    (executable ? 0 : 0x8000000000000000);

}

void *find_free_kernel_region(uint64_t length) {

  length /= 4096;

  uint64_t next = 0;

  uint64_t run_start = 0;
  uint64_t run_length = 0;

  while (1) {
    if (run_length == length)
      return (void *)(run_start * 4096 + 0xffffffffc0000000);
    if (next == 512 * 512)
      //die. TODO: handle this nicer.
      while (1)
        __asm__ ("hlt");
    if (kernel_p1s[next] == 0)
      ++run_length;
    else {
      run_length = 0;
      run_start = next + 1;
    }
    ++next;
  }

}