Re: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP - Russell King

From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: "Nicolas Pitre" <nico@fluxnic.net>,
	"Will Deacon" <Will.Deacon@arm.com>,
	"Måns Rullgård" <mans@mansr.com>,
	lkml <linux-kernel@vger.kernel.org>,
	ak@linux.intel.com, "Andrew Morton" <akpm@linux-foundation.org>,
	sam@ravnborg.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP
Date: Fri, 27 May 2011 09:54:14 +0100	[thread overview]
Message-ID: <20110527085414.GP24876@n2100.arm.linux.org.uk> (raw)
In-Reply-To: <20110527083806.GA21100@e102109-lin.cambridge.arm.com>

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.