linux-kernel - Re: [PATCH] ARM: alignment: setup alignment handler earlier

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Date:	Wed, 7 Sep 2011 17:43:12 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Dave Martin <dave.martin@...aro.org>,
	John Ogness <john.ogness@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: alignment: setup alignment handler earlier

On 7 September 2011 17:28, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Wed, Sep 07, 2011 at 03:40:16PM +0100, Catalin Marinas wrote:
>> On Wed, Sep 07, 2011 at 02:35:04PM +0100, John Ogness wrote:
>> > From 6f3f381800367127dc6430d9b9fa9bd6fc6d8ed0 Mon Sep 17 00:00:00 2001
>> > From: John Ogness <john.ogness@...utronix.de>
>> >
>> > The alignment exception handler is setup fairly late in
>> > the boot process (fs_initcall). However, with newer gcc
>> > versions and the compiler option -fconserve-stack, code
>> > accessing unaligned data is generated in functions that
>> > are called earlier, for example pcpu_dump_alloc_info().
>> > This results in unhandled alignment exceptions during
>> > boot. By setting up the exception handler sooner, we
>> > reduce the window where a compiler may generate code
>> > accessing unaligned data.
>>
>> While this reduces the window and fixes this particular case, it still
>> doesn't solve the problem. We never know when some unaligned access
>> would be generated for some earlier code.
>
> Is the problem even solvable?  There are instructions on ARMv6+ which
> always produce an alignment fault (eg, ldrd) irrespective of strict
> alignment checking.

We can just hope that the compiler isn't that bad as to generate
unaligned accesses with those instructions that are known to fault.
That's what I would consider a compiler bug.

-- 
Catalin
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