David,

From: Russell King - ARM Linux <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>
Date: Sat, 30 Nov 2013 19:15:53 +0000

quoted

so there should be no undesired side effect from this packing.

There is a huge side effect from ever using the packed attribute, in
that the compiler can assume absolutely nothing about the alignment of
any object or sub-object of the type you apply this attribute to.

Even if it is "obvious" that some members will be aligned, the
compiler cannot take advantage of this assumption because this
attribute also means that an array of such elements might have
arbitrary alignment.  So you when you get a pointer to one of these
objects, the compiler has to assume the worst possible case.

This means using 4 byte loads to load a 32-bit quantity, always,
unconditionally, no matter what.

That's why we should do whatever is necessary to align things properly
by hand, and use packed only as the last possible and least desirable
resort.

so far we have two feasible options how to handle batadv_header:

 1) Russells patch
 2) unrolling these 3 bytes of header into every packet structure

I understand your concerns for __packed in generally, but in this case it 
might not be that bad:

 * batadv_header is only used by embedding it in other structs (like 
batadv_unicast_packet, batadv_broadcast_packet ...) which are well aligned - I 
guess there should be no performance problem when accessing through the parent 
structure (like unicast_packet->header.version)
 * there is one exception in batadv_interface_rx() where we use this 
batadv_header directly, but we can easily replace that by using e.g. 
batadv_bcast_packet instead.
 * It's easier to read and maintain than copying these 3 bytes in every other 
structure.

Therefore I'd suggest to pick Russels patch, I can send a fix for 
batadv_interface_rx() later, too.

Thanks,
    Simon