linux-kernel - Re: x86-64: Maintain 16-byte stack alignment

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Date:   Fri, 13 Jan 2017 16:42:51 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: x86-64: Maintain 16-byte stack alignment

On Thu, Jan 12, 2017 at 08:37:18PM -0800, Linus Torvalds wrote:
>
> So then the compiler actually needs to start adding useless instructions
> just to keep the stack 16-byte aligned.

Which it does.  Of course most of the time no extra instructions
are required because there are stack variables, so it's just matter
of adding 8 to the value you're subtracting from rsp.  But it is
probably why gcc assumes that the stack is 16-byte aligned which
triggered my original crash.

Here is an example from the function that was involved in the crash,
without frame pointers:

00000000000001b0 <chacha20_simd>:
 1b0:   41 54                   push   %r12
 1b2:   55                      push   %rbp
 1b3:   48 81 ec f8 00 00 00    sub    $0xf8,%rsp

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt