alignment faults in 3.6

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Fri, Oct 05, 2012 at 03:25:16AM +0100, Mans Rullgard wrote:
> On 5 October 2012 02:56, Rob Herring <robherring2@gmail.com> wrote:
> > This struct is the IP header, so a struct ptr is just set to the
> > beginning of the received data. Since ethernet headers are 14 bytes,
> > often the IP header is not aligned unless the NIC can place the frame at
> > a 2 byte offset (which is something I need to investigate). So this
> > function cannot make any assumptions about the alignment. Does the ABI
> > define structs have some minimum alignment? Does the struct need to be
> > declared as packed or something?
> 
> The ABI defines the alignment of structs as the maximum alignment of its
> members.  Since this struct contains 32-bit members, the alignment for the
> whole struct becomes 32 bits as well.  Declaring it as packed tells gcc it
> might be unaligned (in addition to removing any holes within).

This has come up before in the past.

The Linux network folk will _not_ allow - in any shape or form - for
this struct to be marked packed (it's the struct which needs to be
marked packed) because by doing so, it causes GCC to issue byte loads/
stores on architectures where there isn't a problem, and that decreases
the performance of the Linux IP stack unnecessarily.

I don't think there's going to be a satisfactory answer to this issue.
I think we're going to be stuck between GCC people saying that the
kernel is buggy, and the network people refusing to fix this as they
have done in the past.