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

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Date: Mon, 15 May 2023 17:36:12 +0200
From: Jesper Dangaard Brouer <jbrouer@...hat.com>
To: Larysa Zaremba <larysa.zaremba@...el.com>, bpf@...r.kernel.org
Cc: brouer@...hat.com, Stanislav Fomichev <sdf@...gle.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
 Yonghong Song <yhs@...com>, John Fastabend <john.fastabend@...il.com>,
 KP Singh <kpsingh@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
 Jesse Brandeburg <jesse.brandeburg@...el.com>,
 Tony Nguyen <anthony.l.nguyen@...el.com>,
 Anatoly Burakov <anatoly.burakov@...el.com>,
 Alexander Lobakin <alexandr.lobakin@...el.com>,
 Magnus Karlsson <magnus.karlsson@...il.com>,
 Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net,
 netdev@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND bpf-next 09/15] xdp: Add VLAN tag hint



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@...el.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;
> +}
> +