message: minor optimization by avoiding register collisions
diff: @@ -60,8 +60,8 @@ pub extern "C" fn switch(old_stack: *mut usize, new_stack: usize) {
  // set stack pointer in TSS \n\t\
  call set_current_kernel_stack \n\t\
  // restore context \n\t\
  - pop %r15\n\t\
  - wrfsbaseq %r15\n\t\
  + pop %rax\n\t\
  + wrfsbaseq %rax\n\t\
  pop %r15\n\t\
  pop %r14\n\t\
  pop %r13\n\t\
  