Re: [RFC bpf-next 0/4] Add XDP rx hw hints support performing XDP_REDIRECT

Arthur Fabre
2024-10-04T13:55:15Z

On Fri Oct 4, 2024 at 12:38 PM CEST, Jesper Dangaard Brouer wrote:
> [...]
> >>> There are two different use-cases for the metadata:
> >>>
> >>> * "Hardware" metadata (like the hash, rx_timestamp...). There are only a
> >>> few well known fields, and only XDP can access them to set them as
> >>> metadata, so storing them in a struct somewhere could make sense.
> >>>
> >>> * Arbitrary metadata used by services. Eg a TC filter could set a field
> >>> describing which service a packet is for, and that could be reused for
> >>> iptables, routing, socket dispatch...
> >>> Similarly we could set a "packet_id" field that uniquely identifies a
> >>> packet so we can trace it throughout the network stack (through
> >>> clones, encap, decap, userspace services...).
> >>> The skb->mark, but with more room, and better support for sharing it.
> >>>
> >>> We can only know the layout ahead of time for the first one. And they're
> >>> similar enough in their requirements (need to be stored somewhere in the
> >>> SKB, have a way of retrieving each one individually, that it seems to
> >>> make sense to use a common API).
> >>
> >> Why not have the following layout then?
> >>
> >> +---------------+-------------------+----------------------------------------+------+
> >> | more headroom | user-defined meta | hw-meta (potentially fixed skb format) | data |
> >> +---------------+-------------------+----------------------------------------+------+
> >> ^ ^
> >> data_meta data
> >>
> >> You obviously still have a problem of communicating the layout if you
> >> have some redirects in between, but you, in theory still have this
> >> problem with user-defined metadata anyway (unless I'm missing
> >> something).
> >>
>
> Hmm, I think you are missing something... As far as I'm concerned we are
> discussing placing the KV data after the xdp_frame, and not in the XDP
> data_meta area (as your drawing suggests). The xdp_frame is stored at
> the very top of the headroom. Lorenzo's patchset is extending struct
> xdp_frame and now we are discussing to we can make a more flexible API
> for extending this. I understand that Toke confirmed this here [3]. Let
> me know if I missed something :-)
>
> [3] https://lore.kernel.org/all/874j62u1lb.fsf@toke.dk/
>
> As part of designing this flexible API, we/Toke are trying hard not to
> tie this to a specific data area. This is a good API design, keeping it
> flexible enough that we can move things around should the need arise.

+1. And if we have an API for doing this for user-defined metadata, it
seems like we might as well use it for hardware metadata too.

With something roughly like:

*val get(id)

set(id, *val)

with pre-defined ids for hardware metadata, consumers don't need to know
the layout, or where / how the data is stored.

Under the hood we can implement it however we want, and change it in the
future.

I was initially thinking we could store hardware metadata the same way
as user defined metadata, but Toke and Lorenzo seem to prefer storing it
in a fixed struct.