[xdp-hints] Re: [RFC bpf-next v2 03/11] xsk: Support XDP_TX_METADATA_LEN

[Stanislav Fomichev <sdf@google.com>]

On Thu, Jun 22, 2023 at 2:11 AM Jesper D. Brouer <netdev@brouer.com> wrote:
>
>
> This needs to be reviewed by AF_XDP maintainers Magnus and Bjørn (Cc)
>
> On 21/06/2023 19.02, Stanislav Fomichev wrote:
> > For zerocopy mode, tx_desc->addr can point to the arbitrary offset
> > and carry some TX metadata in the headroom. For copy mode, there
> > is no way currently to populate skb metadata.
> >
> > Introduce new XDP_TX_METADATA_LEN that indicates how many bytes
> > to treat as metadata. Metadata bytes come prior to tx_desc address
> > (same as in RX case).
>
>  From looking at the code, this introduces a socket option for this TX
> metadata length (tx_metadata_len).
> This implies the same fixed TX metadata size is used for all packets.
> Maybe describe this in patch desc.

I was planning to do a proper documentation page once we settle on all
the details (similar to the one we have for rx).

> What is the plan for dealing with cases that doesn't populate same/full
> TX metadata size ?

Do we need to support that? I was assuming that the TX layout would be
fixed between the userspace and BPF.
If every packet would have a different metadata length, it seems like
a nightmare to parse?