Re: alignment faults in 3.6
flat view
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2012-10-11 16:59:49
Also in:
linux-arm-kernel
Possibly related (same subject, not in this thread)
- 2012-10-12 · alignment faults in 3.6 · Ben Hutchings <hidden>
- 2012-10-12 · alignment faults in 3.6 · David Laight <hidden>
- 2012-10-12 · alignment faults in 3.6 · Ben Hutchings <hidden>
- 2012-10-12 · alignment faults in 3.6 · bcrl@kvack.org (Benjamin LaHaise)
- 2012-10-12 · alignment faults in 3.6 · Eric Dumazet <hidden>
On 11 October 2012 11:32, Russell King - ARM Linux [off-list ref] wrote:
On Thu, Oct 11, 2012 at 12:22:06PM +0200, Eric Dumazet wrote:quoted
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.
It got better with ARMv6 where LDRD/STRD only require 4 byte alignment (the only 8 byte alignment is required by LDREXD/STREXD). But on ARMv5 LDRD/STRD 8 byte alignment is indeed required (otherwise unpredictable). -- Catalin