Re: [PATCH v20 25/25] app/pdump: preserve VLAN tags in captured packets

Bruce Richardson
2026-03-25T09:12:53Z

On Wed, Mar 25, 2026 at 08:41:39AM +0100, Morten Brørup wrote:
> > From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> > Sent: Tuesday, 24 March 2026 18.12
> >
> > On Mon, 16 Mar 2026 16:55:29 +0100
> > Morten Brørup <mb@smartsharesystems.com> wrote:
> >
> > > >
> > > > This is an example of something I previously flagged. Like with
> > real
> > > > hardware, I think the PMD should be inserting the VLAN tag into the
> > > > packet
> > > > as part of the Tx function, not the prepare function.
> > >
> > > Agree with Bruce on this.
> > > For simple stuff like VLAN offload, applications should not be
> > required to call tx_prep first.
> > >
> > > However, the Tx function is supposed to not modify the packets;
> > relevant when refcnt > 1.
> > >
> > > Instead of modifying the packet data to insert/strip the VLAN tag,
> > > perhaps the driver can split the write/read operation into multiple
> > write/read operations:
> > > 1. the Ethernet header
> > > 2. the VLAN tag
> > > 3. the remaining packet data
> > >
> > > I haven't really followed the pcap driver, so maybe my suggestion
> > doesn't make sense.
> >
> > The prepare code and VLAN was copied from virtio.
> > I assume virtio is widely used already.
>
> OK, that makes it harder to object to.
>

Yes, but I also believe that the topic was not previously discussed and
that the virtio driver may be wrong in how it behaves.

I still think, for consistency with HW drivers, SW drivers should do the
tagging in the Tx function.

I also think that we should provide a DPDK-lib level helper function (be it in
ethdev or elsewhere) for doing this sort of thing for all drivers. That way
we can put in the necessary copying of packets with refcnt > 1 and have it
apply globally.

/Bruce