[SMP] SMP synchronization and scheduling

kernel/cpu/cpu.c

@@ -62,7 +62,9 @@ void cpu_init()
   all_ap_started = 1;
 
   vmm_set_page(0, TRAMPOLINE_ADDR, 0, 0);
-  pmm_free(page);
+  // Keep the GDT mapped for now
+  /* vmm_set_page(0, TRAMPOLINE_GDT, 0, 0); */
+  /* pmm_free(page); */
 
   debug_info("CPU - Status\n");
   for(unsigned int i = 0; i < num_cpu; i++)

kernel/cpu/smp.c

@@ -49,5 +49,7 @@ void ap_start()
   while(!all_ap_started);
   cpu_t *cpu = get_cpu();
   debug_ok("STARTED CPU:%x\n", cpu->id);
+  schedule();
+  debug_error("PANIC: SMP - This line should be unreachable\n");
   for(;;)asm("hlt");
 }

kernel/cpu/sync.S

@@ -0,0 +1,16 @@
+.intel_syntax noprefix
+.global spin_lock
+spin_lock:
+  mov rdx, rdi
+  mov rax, 0x0
+  mov rcx, 0x1
+.retry:
+  xor rax, rax
+  lock cmpxchg [rdx], cl
+  jnz .retry
+  ret
+
+.global spin_unlock
+spin_unlock:
+  movq [rdi], 0x0
+  ret