linux-kernel - Re: [PATCH v2 03/44] x86/asm/head: rename 'stack_start' -> 'initial

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Date:	Sat, 6 Aug 2016 09:15:30 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Nilay Vaish <nilayvaish@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, x86 <x86@...nel.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH v2 03/44] x86/asm/head: rename 'stack_start' -> 'initial_stack'

On Sat, Aug 6, 2016 at 1:25 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Fri, Aug 05, 2016 at 11:01:57AM -0500, Josh Poimboeuf wrote:
>> The 8 should be changed to SIZEOF_PTREGS in a later patch
>> ("x86/asm/head: standardize the end of the stack for idle tasks").
>
> But SIZEOF_PTREGS is 21*8. I don't understand.

This patch is only for the boot cpu's idle thread.  All other kernel
threads, including idle threads for the secondary cpus, already have
the pt_regs area reserved.  My best guess for the current 8 byte
padding is to make sure thread_info is calculated properly (by masking
off the low bits from RSP).

Also, this fix should be applied to 32-bit, but make sure to account
for TOP_OF_KERNEL_STACK_PADDING.

--
Brian Gerst