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

Stephen Hemminger
2026-03-25T17:33:15Z

On Wed, 25 Mar 2026 16:22:45 +0000
Bruce Richardson <bruce.richardson@intel.com> wrote:

> On Wed, Mar 25, 2026 at 09:19:21AM -0700, Stephen Hemminger wrote:
> > On Wed, 25 Mar 2026 10:36:56 +0100
> > Morten Brørup <mb@smartsharesystems.com> wrote:
> >
> > > If an application clones packets instead of copying them, it is probably for performance reasons.
> > > If the drivers start copying those clones, it may defeat the performance purpose.
> > >
> > > <brainstorming>
> > > Maybe segmentation can be used instead of copying the full packet:
> > > Make the "copy" packet of two (or more) segments, where the header is copied into a new mbuf (where the VLAN tag is added), and the remaining part of the packet uses an indirect mbuf referring to the "original" packet at the offset after the header.
> > > </brainstorming>
> > >
> > > Furthermore...
> > > If drivers start copying packets in the Tx function, the Tx queue should have its own mbuf pool to allocate these mbufs from.
> > > Drivers should not steal mbufs from the pools used by the packets being transmitted.
> > > E.g. if a segmented packet has a small mbuf for the first few bytes, followed by a large mbuf (from another pool) for the remaining bytes.
> > > Or if the "original" mbuf comes from a mempool allocated on different CPU socket, the "copy" would too.
> >
> >
> > The problem with the Tx function is how backpressure gets handled.
> > Not sure that it is documented well enough that if a packet is not sent
> > due to backpressure, the mbuf in the array may still have been replaced.
>
> Most drivers should be able to check for space in a Tx ring, or whatever
> other backpressure mechanism is being used, before modifying a buffer.
>
> /Bruce

The wording in tx_prepare documentation mentions checksums requirement but not vlans.