alignment faults in 3.6

On Fri, Oct 05, 2012 at 09:20:56AM +0100, Mans Rullgard wrote:

On 5 October 2012 08:12, Russell King - ARM Linux
[off-list ref] wrote:

quoted

On Fri, Oct 05, 2012 at 03:25:16AM +0100, Mans Rullgard wrote:

quoted

On 5 October 2012 02:56, Rob Herring [off-list ref] wrote:

quoted

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.

Which architectures?  I have never seen anything like that.

Does it matter?  I'm just relaying the argument against adding __packed
which was used before we were forced (by the networking folk) to implement
the alignment fault handler.