[xdp-hints] Re: [PATCH RESEND bpf-next 09/15] xdp: Add VLAN tag hint

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 12/05/2023 17.26, Larysa Zaremba wrote:
> Implement functionality that enables drivers to expose VLAN tag
> to XDP code.
> 
> Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> ---
[...]

> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 41e5ca8643ec..eff21501609f 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -738,6 +738,30 @@ __bpf_kfunc int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, u32 *hash,
>   	return -EOPNOTSUPP;
>   }
>   

Remember below becomes part of main documentation on HW metadata hints:
  - https://kernel.org/doc/html/latest/networking/xdp-rx-metadata.html

Hint compiling locally I use:
  make SPHINXDIRS="networking" htmldocs

> +/**
> + * bpf_xdp_metadata_rx_ctag - Read XDP packet inner vlan tag.

Is bpf_xdp_metadata_rx_ctag a good function name for the inner vlan tag?
Like wise below "stag".

I cannot remember if the C-tag or S-tag is the inner or outer vlan tag.

When reading BPF code that use these function names, then I would have
to ask Google for help, or find-and-read this doc.

Can we come-up with a more intuitive name, that e.g. helps when reading
the BPF-prog code?

> + * @ctx: XDP context pointer.
> + * @vlan_tag: Return value pointer.
> + *

IMHO right here, there should be a description.

E.g. for what a VLAN "tag" means.  I assume a "tag" isn't the VLAN id,
but the raw VLAN tag that also contains the prio numbers etc.

It this VLAN tag expected to be in network-byte-order ?
IMHO this doc should define what is expected (and driver devel must
follow this).

> + * Returns 0 on success or ``-errno`` on error.
> + */
> +__bpf_kfunc int bpf_xdp_metadata_rx_ctag(const struct xdp_md *ctx, u16 *vlan_tag)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +/**
> + * bpf_xdp_metadata_rx_stag - Read XDP packet outer vlan tag.
> + * @ctx: XDP context pointer.
> + * @vlan_tag: Return value pointer.
> + *
> + * Returns 0 on success or ``-errno`` on error.

IMHO we should provide more guidance to expected return codes, and what
they mean.  IMHO driver developers must only return codes that are
described here, and if they invent a new, add it as part of their patch.

See, formatting in bpf_xdp_metadata_rx_hash and check how this gets
compiled into HTML.


> + */
> +__bpf_kfunc int bpf_xdp_metadata_rx_stag(const struct xdp_md *ctx, u16 *vlan_tag)
> +{
> +	return -EOPNOTSUPP;
> +}
> +