netdev - Re: alignment faults in 3.6

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Message-ID: <20121011103257.GO4625@n2100.arm.linux.org.uk>
Date:	Thu, 11 Oct 2012 11:32:57 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Måns Rullgård <mans@...sr.com>,
	Jon Masters <jonathan@...masters.org>, netdev@...r.kernel.org,
	David Laight <David.Laight@...LAB.COM>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: alignment faults in 3.6

On Thu, Oct 11, 2012 at 12:22:06PM +0200, Eric Dumazet wrote:
> I took a look, and I dont see why/how gcc could use a ldm instruction
> 
> Doing so assumed the alignment of the structure was 8 bytes, but its
> not.
> 
> Networking stack mandates that IP headers are aligned on 4 bytes
> boundaries, not 8 bytes.

Err, no.  ldm is "load multiple" not "load double".  It loads multiple
32-bit registers, and its requirement for non-faulting behaviour is for
the pointer to be 4 byte aligned.  However, "load double" requires 8
byte alignment.
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