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

[Jesper Dangaard Brouer <hawk@kernel.org>]

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.

--Jesper