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

[Marcus Wichelmann <marcus.wichelmann@hetzner-cloud.de>]

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'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