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

Arthur Fabre
2024-10-04T14:29:27Z

On Fri Oct 4, 2024 at 4:18 PM CEST, Lorenzo Bianconi wrote:
> On Oct 04, Jesper Dangaard Brouer wrote:
> > On 04/10/2024 15.55, Arthur Fabre wrote:
> > > 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.
> >
> > If the API hide the actual location then we can always move things
> > around, later. If your popcnt approach is fast enough, then IMO we
> > don't need a fixed struct for hardware metadata.
>
> +1. I am fine with the KV approach for nic metadata as well if it is fast enough.

Great! That's simpler. I should have something for Jesper to benchmark
on Monday.

> If you want I can modify my series to use kfunc sto store data after xdp_frame
> and then you can plug the KV encoding. What do you think? Up to you.

Thanks for the offer! That works for me :)