Why do we reserve 8 bytes on top of the boot time stack?

Hello,

We reserve top 8 bytes of the boot stack in the arch/x86/kernel/head_64.S:

GLOBAL(stack_start)
.quad init_thread_union+THREAD_SIZE-8

So, the question is in subject. Why? I've found definition of the TOP_OF_KERNEL_STACK_PADDING macro arch/x86/include/asm/thread_info.h, but it is zero for the x86_64.

Thank you.