From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen-OTpzqLSitTUnbdJkjeBofR2eb7JE58TQ@public.gmane.org>
Subject: Re: [RFC 06/10] virtio: use software vlan stripping
Date: Wed, 27 Aug 2014 11:04:52 -0700
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To: "Ouyang, Changchun" <changchun.ouyang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
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On Wed, 27 Aug 2014 05:42:09 +0000
"Ouyang, Changchun" <changchun.ouyang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> wrote:

> 
> > -----Original Message-----
> > From: Stephen Hemminger [mailto:stephen-OTpzqLSitTUnbdJkjeBofR2eb7JE58TQ@public.gmane.org]
> > Sent: Wednesday, August 27, 2014 12:24 AM
> > To: Ouyang, Changchun
> > Cc: dev-VfR2kkLFssw@public.gmane.org
> > Subject: Re: [RFC 06/10] virtio: use software vlan stripping
> > 
> > On Tue, 26 Aug 2014 08:37:11 +0000
> > "Ouyang, Changchun" <changchun.ouyang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> wrote:
> > 
> > > Hi Stephen,
> > >
> > > Would you please describe the use scenario for the front end rx vlan strip
> > and tx vlan insertion?
> > > In our current implementation, backend will strip vlan tag for RX, and insert
> > vlan tag for TX.
> > >
> > > Thanks
> > > Changchun
> > 
> > First, we don't have to do software VLAN strip on our backend if we do this.
> > And this way we can always use VLAN insert on transmit. Otherwise you
> > have to introduce special case because there is no DPDK API to determine if
> > device does or does not do VLAN handling.
> > 
> 
> How the virtio frontend tell backend whether it has software VLAN strip feature or not?
> It seems no feature bit to negotiate it.
> 
> Thanks
> Changchun
> 

All other drivers have VLAN stripping as option under rte_eth_rxmode / hw_vlan_strip.
There is negotiation in DPDK, it is strictly ask for feature and either get it or an error.
Therefore we went ahead and emulated the feature in software.