linux-kernel - Re: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT

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Message-ID: <20110527085414.GP24876@n2100.arm.linux.org.uk>
Date:	Fri, 27 May 2011 09:54:14 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Nicolas Pitre <nico@...xnic.net>,
	Will Deacon <Will.Deacon@....com>,
	Måns Rullgård <mans@...sr.com>,
	lkml <linux-kernel@...r.kernel.org>, ak@...ux.intel.com,
	Andrew Morton <akpm@...ux-foundation.org>, sam@...nborg.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM: Do not allow unaligned accesses when
	CONFIG_ALIGNMENT_TRAP

On Fri, May 27, 2011 at 09:38:08AM +0100, Catalin Marinas wrote:
> OK, I tried this now:
> 
> -fconserve-stack: we get unaligned accesses on the stack because the
> newer versions of gcc turned unaligned accesses on by default.
> 
> -fconserve-stack -mno-unaligned-access: the stack variables are aligned.
> We probably get the benefit of -fconserve-stack as well.
> 
> So as per the initial post in this thread, we could have
> -mno-unaligned-access on ARM always on (when CONFIG_ALIGNMENT_TRAP). As
> Nicolas suggested, we could compile some files with -munaligned-access
> (and maybe -fno-conserve-stack).
> 
> I raised this with the gcc guys so they are looking into it. But it
> really doesn't look like a gcc bug as long as -mno-unaligned-access is
> taken into account.

Ok, we need to check one last thing, and that's what the behaviour is
with -mno-unaligned-access and packed structures (such as the ethernet
header).  If it makes no difference, then I suggest we always build
with -mno-unaligned-access.
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