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

[Olivier Matz <olivier.matz@6wind.com>]

Hi Bruce,

On Mon, Jul 01, 2019 at 02:38:43PM +0100, Bruce Richardson wrote:
> 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.

Maybe you missed that part of my previous answer, I'm copy it again here:

  > Although this is an ABI break, the network structures are all being renamed
  > in this release, and a deprecation notice was previously posted for it.

  Yes, but the network renaming is identified in the release note as an
  API break, not an ABI break.

I thought we agreed to limit ABI breakages to cases where there is no
other solution. Here, this is surely a "small" ABI breakage, but I
suppose there is a way to do differently.

If we really want to do that way, it's better to announce it as an ABI
break.

Olivier