From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [PATCH 2/5] mbuf: use the reserved 16 bits for
 double vlan
Date: Tue, 26 May 2015 08:35:05 -0700
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On Tue, 26 May 2015 15:02:51 +0000
"Ananyev, Konstantin" <konstantin.ananyev@intel.com> wrote:

> Hi Stephen,
> 
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Stephen Hemminger
> > Sent: Tuesday, May 26, 2015 3:55 PM
> > To: Zhang, Helin
> > Cc: dev@dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH 2/5] mbuf: use the reserved 16 bits for double vlan
> > 
> > On Tue, 26 May 2015 16:36:37 +0800
> > Helin Zhang <helin.zhang@intel.com> wrote:
> > 
> > > Use the reserved 16 bits in rte_mbuf structure for the outer vlan,
> > > also add QinQ offloading flags for both RX and TX sides.
> > >
> > > Signed-off-by: Helin Zhang <helin.zhang@intel.com>
> > 
> > Yet another change that is much needed, but breaks ABI compatibility.
> 
> Why do you think it breaks ABI compatibility?
> As I can see, it uses field that was reserved.
> Konstantin

Because an application maybe assuming something or reusing the reserved fields.
Yes, it would be dumb of application to do that but from absolute ABI point
of view it is a change.