Re: alignment faults in 3.6

On Thursday 11 October 2012, Eric Dumazet wrote:

On Thu, 2012-10-11 at 11:32 +0100, Russell King - ARM Linux wrote:

quoted

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.

So if you have an alignment fault, thats because IP header is not
aligned on 4 bytes ?

If so a driver is buggy and must be fixed.

Please send us full stack trace

Rob Herring as the original reporter has dropped off the Cc list, adding
him back.

I assume that the calxeda xgmac driver is the culprit then. It uses
netdev_alloc_skb() rather than netdev_alloc_skb_ip_align() in
xgmac_rx_refill but it is not clear whether it does so intentionally
or by accident.

	Arnd