[OSDev] How to switch to long mode in x86 CPUs?

Hello there,

I am trying to develop my own, simple operating system to learn more
about how kernels work and low level stuff like that.  However, I am
stuck at setting up paging while switching long mode (64-bit protected
mode) in x86 processors.

The assembly code I currently have:

#define PG_START 0x000000000
#define MSR_EFER 0xc0000080

.section .bss, "aw", @nobits
.align 4096
pml4_tbl:
  .skip 4096
pdpt_tbl:
  .skip 4096

.text
.globl _mboot_start
_mboot_start:
  /* GRUB executes this code in 32-bit protected mode. */

  /* Write (pdpt_tbl | 0x3) to the first 8 bytes of pml4_tbl */
  movl $pdpt_tbl, %eax
  orl $0x3, %eax
  movl $pml4_tbl, %edi
  movl %eax, (%edi)
  xorl %eax, %eax
  movl %eax, 4(%edi)
 
  movl $pdpt_tbl, %edi
  movl $PG_START, %eax
  /* 0x83 = 0b10000011; flags: present, writable, upervisor-only,
     1GB huge page */
  movl $0x83, (%edi)
  movl %eax, 4(%edi)

  /* Enable Physical Address Extension (PAE) */
  movl %cr4, %eax
  btsl $5, %eax
  movl %eax, %cr4

  /* Load the address of the PML4 table into %cr3 */
  movl $pml4_tbl, %edi
  movl %edi, %cr3

  /* Enable long mode */
  movl $MSR_EFER, %ecx
  rdmsr
  btsl $8, %eax
  wrmsr

  /* Enable paging */
  movl %cr0, %eax
  btsl $31, %eax
  movl %eax, %cr0

  /* Jump to 64-bit code */
  ljmpl $0x08, $long_mode_entry
.loop:
  hlt
  jmp .loop

long_mode_entry:
  .code64
  xorw %ax, %ax
  movw %ax, %ds
  movw %ax, %es
  movw %ax, %fs
  movw %ax, %gs
  movw %ax, %ss
 
  callq kmain
  callq kabort

I am not sure what is wrong, but when I run my kernel in
qemu-system-x86_64, it causes a triple fault when trying to jump to the
long mode code.  After a lot of debugging, I am sure that the issue is
with paging, because removing the ljmpl and paging instructions do not
cause any further errors and the kernel runs fine in 32-bit mode.

If anyone knows what is wrong with this code, please let me know.  Any
help will be appreciated!

--
Ar Rakin