x86-64 compatible call to user code

jump_to_user_code.c

/** @brief Jump back to user code
 *
 * This function runs the user code after stopping it just as if
 * it was a return from a procedure.
 *
 * @return 0 in any case
 */
static inline int jump_to_user_code(size_t ep, size_t stack)
{
#ifdef CONFIG_X86_64
	size_t *argc = (size_t*) stack;
	size_t *argv = (size_t*) stack + 1;
	size_t *env = (size_t*) stack + 2;

	stack += 3 * sizeof(size_t);
#elif defined (CONFIG_X86_32)
	asm volatile ("mov %0, %%ds; mov %0, %%fs; mov %0, %%gs; mov %0, %%es" :: "r"(0x23)); // update segment registers
#endif

	asm volatile ("push $0x23; push %0; push $0x1B; push %1" :: "r"(stack), "r"(ep)); // fake stack, see Intel Reference Manual, Vol 1, 6.3.6

#ifdef CONFIG_X86_64
	asm volatile ("lretq" :: "S" (*argc), "D" (*argv), "d" (*env) : "cc"); // far return to user level code and save arguments to registers
#elif defined (CONFIG_X86_32)
	asm volatile ("lret" ::: "cc"); // far return to user level code
#endif

	return 0;
}