[xdp-hints] Re: XDP Redirect and TX Metadata

[Florian Kauer <florian.kauer@linutronix.de>]

Hi Marcus,

On 1/14/25 17:47, Marcus Wichelmann wrote:
> Am 12.02.24 um 15:35 schrieb Florian Kauer:
>> On 12.02.24 14:41, Toke Høiland-Jørgensen wrote:
>>> Florian Kauer <florian.kauer@linutronix.de> writes:
>>>
>>>> 2. For the RX side, the metadata is documented as "XDP RX Metadata"
>>>> (https://docs.kernel.org/networking/xdp-rx-metadata.html), while for
>>>> TX it is "AF_XDP TX Metadata"
>>>> (https://www.kernel.org/doc/html/next/networking/xsk-tx-metadata.html).
>>>> That seems to imply that TX metadata only works for AF_XDP, but not
>>>> for direct redirection. Is there a reason for that?
>>>
>>> Well, IIRC, AF_XDP was the most pressing use case, and no one has gotten
>>> around to extending this to the regular XDP forwarding path yet.
>>
>> Ok, that is fine. I had the fear that there is some fundamental problem
>> that prevents to implement this.
> 
> Hi,
> are there any updates on this? I'm currently looking into this as well.

I am still interested, but have no implementation planned short- or mid-term.
So, looking forward to your implementation :-)

Greetings,
Florian

> 
> I'd like to have a way to enable the TX checksum offload when redirecting from
> one device to another.
> Stanislav Fomichev already implemented [1] the TX offload support for the AF_XDP
> use case (thanks for that), but for now, this cannot be used for "regular"
> redirects.
> 
> I'm currently in a position where I can invest some work into this, but figured
> it would make sense to ask you first:
> 
> Do you already have concrete plans or ideas in mind, how the API to trigger the
> TX offloads should look like?
> I have seen the talk [2] from Jesper about this, but I'm not sure if the
> proposals in there are still up to date.
> 
> I think it could be possible to introduce a program flag, just like
> `BPF_F_XDP_HAS_FRAGS`, and if this flag is set, interpret a part of the
> metadata area as a `struct xsk_tx_metadata`. Then, the code to apply the
> offloads from that struct when xmit-ing the frame could be reused, as it
> is already implemented in `mlx5e_xmit_xdp_frame` for example.
> But the "xsk" in the struct name may be a bit confusing. :/
> 
> Do you think this could work or could you guide me into a direction that may
> have a chance to be upstreamable? Also, is there any recent work on this that
> I should know off?
> 
> Thanks!
> 
> Marcus Wichelmann
> Hetzner Cloud GmbH
> 
> [1] https://lore.kernel.org/bpf/20231127190319.1190813-3-sdf@google.com/
> [2] https://lpc.events/event/16/contributions/1362/attachments/1056/2017/xdp-hints-lpc2022.pdf
>