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

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Message-ID: <20160129130042.GA12414@localhost>
Date:	Fri, 29 Jan 2016 19:00:42 +0600
From:	Alexnader Kuleshov <kuleshovmail@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: 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.