Fix stack trace, WIP usermode

arch/x86_64/src/start.rs

@@ -137,6 +137,28 @@ pub unsafe extern fn kstart() -> ! {
     kmain();
 }
 
+unsafe fn usermode(ip: usize, sp: usize) {
+    // Test usermode
+    asm!("xchg bx, bx
+        mov rax, 0x2B
+        mov ds, ax
+        mov es, ax
+        mov fs, ax
+        mov gs, ax
+
+        push rax
+        push rbx
+        pushfq
+        mov rax, 0x23
+        push rax
+        push rcx
+        iretq"
+        :
+        : "{rbx}"(sp), "{rcx}"(ip)
+        : "rax", "rbx", "rcx", "sp"
+        : "intel", "volatile");
+}
+
 /// Entry to rust for an AP
 pub unsafe extern fn kstart_ap(stack_start: usize, stack_end: usize) -> ! {
     {