[dpdk-dev] [PATCH v4 5/8] net/ether: mark ethernet addresses as being 2-byte aligned

[Ananyev, Konstantin]

> > > -----Original Message-----
> > > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Stephen Hemminger
> > > Sent: Wednesday, June 5, 2019 7:10 PM
> > > To: dev at dpdk.org
> > > Cc: Richardson, Bruce <bruce.richardson at intel.com>; Stephen Hemminger
> > > <stephen at networkplumber.org>; Andrew Rybchenko
> > > <arybchenko at solarflare.com>
> > > Subject: [dpdk-dev] [PATCH v4 5/8] net/ether: mark ethernet addresses
> > > as being 2-byte aligned
> > >
> > > From: Bruce Richardson <bruce.richardson at intel.com>
> > >
> > > 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, we can just inform the compiler of this assumption to remove
> > > the warnings and allow us to always access the addresses using 16-bit
> > operations.
> > >
> > > Signed-off-by: Bruce Richardson <bruce.richardson at intel.com>
> > > Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
> > > Reviewed-by: Andrew Rybchenko <arybchenko at solarflare.com>
> > > ---
> > >  lib/librte_net/rte_ether.h | 11 ++++++-----
> > >  1 file changed, 6 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/lib/librte_net/rte_ether.h b/lib/librte_net/rte_ether.h
> > > index feb35a33c94b..d7b76ddf63eb 100644
> > > --- a/lib/librte_net/rte_ether.h
> > > +++ b/lib/librte_net/rte_ether.h
> > > @@ -58,7 +58,8 @@ extern "C" {
> > >   * See http://standards.ieee.org/regauth/groupmac/tutorial.html
> > >   */
> > >  struct rte_ether_addr {
> > > -	uint8_t addr_bytes[RTE_ETHER_ADDR_LEN]; /**< Addr bytes in tx order
> > */
> > > +	uint8_t addr_bytes[RTE_ETHER_ADDR_LEN] __rte_aligned(2);
> > > +	/**< Addr bytes in tx order */
> > >  } __attribute__((__packed__));
> >
> > Hmm, that would change layout of any struct/union that has struct
> > rte_ether_addr inside.
> > So seems like implicit ABI breakage to me.
> > Konstantin
> >
> 
> I suppose it could, though only if you had the structure starting at an odd byte
> inside the larger structure.

Yes.

>  Personally, I'd think it should be ok in just about
> all cases, as I'd find it hard to imagine when you wouldn't have this, but
> technically I suppose it's a break.

I also don't expect it to be a common case, but it is not totally impossible.
So I still think it qualifies as ABI change and we need to follow our own ABI
breakage policy here. 

> For real packet heads the fields will always be
> two-byte aligned so there is no issue.
>