[dpdk-dev] [PATCH 2/5] mbuf: use the reserved 16 bits for double vlan

[Zhang, Helin]

> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Tuesday, May 26, 2015 11:46 PM
> To: Stephen Hemminger
> Cc: Zhang, Helin; dev at dpdk.org
> Subject: RE: [dpdk-dev] [PATCH 2/5] mbuf: use the reserved 16 bits for
> double vlan
> 
> 
> 
> > -----Original Message-----
> > From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> > Sent: Tuesday, May 26, 2015 4:35 PM
> > To: Ananyev, Konstantin
> > Cc: Zhang, Helin; dev at dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH 2/5] mbuf: use the reserved 16 bits for
> > double vlan
> >
> > On Tue, 26 May 2015 15:02:51 +0000
> > "Ananyev, Konstantin" <konstantin.ananyev at intel.com> wrote:
> >
> > > Hi Stephen,
> > >
> > > > -----Original Message-----
> > > > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Stephen
> > > > Hemminger
> > > > Sent: Tuesday, May 26, 2015 3:55 PM
> > > > To: Zhang, Helin
> > > > Cc: dev at 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 at 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 at 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.
> 
> But properly behaving application, shouldn't do that right?
> And for misbehaving ones, why should we care about them?
For any reserved bits, I think all application users should avoid touching it,
as it is reserved for future use, or some special reason. Otherwise,
un-predicted behavior can be expected.

Regards,
Helin

> 
> > Yes, it would be dumb of application to do that but from absolute ABI
> > point of view it is a change.
> 
> So, in theory,  even adding a new field to the end of rte_mbuf is an ABI
> breakage?
> Konstantin