Re: [PATCH bpf-next V2 0/7] xdp: Allow BPF to set RX hints for XDP_REDIRECTed packets

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> On Fri, 18 Jul 2025 12:56:46 +0200 Jesper Dangaard Brouer wrote:
> > >> Thanks for the feedback. I can see why you'd be concerned about adding
> > >> another adhoc scheme or making xdp_frame grow into a "para-skb".
> > >>
> > >> However, I'd like to frame this as part of a long-term plan we've been
> > >> calling the "mini-SKB" concept. This isn't a new idea, but a
> > >> continuation of architectural discussions from as far back as [2016].  
> > > 
> > > My understanding is that while this was floated as a plan by some,
> > > nobody came up with a clean way of implementing it.  
> > 
> > I can see why you might think that, but from my perspective, the
> > xdp_frame *is* the implementation of the mini-SKB concept. We've been
> > building it incrementally for years. It started as the most minimal
> > structure possible and has gradually gained more context (e.g. dev_rx,
> > mem_info/rxq_info, flags, and also uses skb_shared_info with same layout
> > as SKB).
> 
> My understanding was that just adding all the fields to xdp_frame was
> considered too wasteful. Otherwise we would have done something along
> those lines ~10 years ago :S

Hi Jakub,

sorry for the late reply.
I am completely fine to redesign the solution to overcome the problem but I
guess this feature will allow us to improve XDP performance in a common/real
use-case. Let's consider we want to redirect a packet into a veth and then into
a container. Preserving the hw metadata performing XDP_REDIRECT will allow us
to avoid recalculating the checksum creating the skb. This will result in a
very nice performance improvement.
So I guess we should really come up with some idea to add this missing feature.

Regards,
Lorenzo

> 
> > This patch is simply the next logical step in that existing evolution:
> > adding hardware metadata to make it more capable, starting with enabling
> > XDP_REDIRECT offloads. The xdp_frame is our mini-SKB, and this patchset
> > continues its evolution.
> 
> I thought one of the goals for mini-skb was to move the skb allocation
> out of the drivers. The patches as posted seem to make it the
> responsibility of the XDP program to save the metadata. If you're
> planning to make drivers populate this metadata by default - why add
> the helpers.
> 
> Again, I just don't understand how these logically fit into place
> vis-a-vis the existing metadata "get" callbacks.

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