Re: [dpdk-dev] [PATCH] ether: mark ethernet addresses as being 2-byte aligned

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Jul 01, 2019 at 03:11:12PM +0200, Olivier Matz wrote:
> Hi Bruce,
> 
> On Thu, May 16, 2019 at 04:54:57PM +0100, Bruce Richardson wrote:
> > When including the rte_ether.h header in applications with warnings
> > enabled, a warning was given because of the assumption of 2-byte alignment
> > of ethernet addresses when processing them.
> > 
> > .../include/rte_ether.h:149:2: warning: converting a packed ‘const
> >   struct ether_addr’ pointer (alignment 1) to a ‘unaligned_uint16_t’
> >   {aka ‘const short unsigned int’} pointer (alignment 2) may result in
> >   an unaligned pointer value [-Waddress-of-packed-member]
> > 149 |  const unaligned_uint16_t *ea_words = (const unaligned_uint16_t *)ea;
> >     |  ^~~~~
> > 
> > Since ethernet addresses should always be aligned on a two-byte boundary,
> 
> I'm a bit reserved about this last assumption. The ethernet address
> structure may be used in a private structure, whose alignment is 1. Are
> we sure that there is no (funny) protocol that carries unaligned
> ethernet addresses?
> 
> Shouldn't we change the definition of unaligned_uint16_t instead?
> Or change the rte_is_broadcast_ether_addr() function?
> 

We could, but I believe the correct behaviour is to make the addresses
always 2-byte aligned, unless someone actually has a real-world case where
there is a protocol that doesn't have the data 2-byte aligned.

/Bruce