Just to clarify, I'm talking about 'lret' not the normal 'ret' instruction. I'm new to x86 and I don't know if this is required in all cases, but I think it's required in most. I got my information from http://docs.oracle.com/cd/E19620-01/805-4693/instructionset-68/index.html but I think the book you mentioned is more specific. 

Here is a more detailed description of my problem:

asm volatile (
  "movabsq	$1f, %%rax	\n"
  "pushq	%%rbx		\n"
  "pushq	%%rax		\n"
  "lretq 			\n"
  "1: 				\n"
    :
    : "b" (KERNEL_CS_64)
    : "rax", "memory");

The code above is causing the problem. What it is supposed to do is to reload the code segment register. Therefor the address of the gdt is pushed as segment selector and the address of the instruction directly following the 'lret' is pushed as return address. The 'lret' instruction then reloads CS and jumps just to the next instruction. This works perfectly fine in qemu but causes trouble in gem5. This is because the segment selector and the return address are still on the stack. The 'ret' of the function this code is part of then returns to the segment selector which results in a crash because it just contains data.