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

[Jesper Dangaard Brouer]

On 15/05/2023 18.09, Larysa Zaremba wrote:
> On Mon, May 15, 2023 at 05:36:12PM +0200, Jesper Dangaard Brouer wrote:
> > 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@xxxxxxxxx>
> > > ---
> > [...]
> >
> > > 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?
>
> Well, my reasoning for such naming is that if someone can configure s-tag
> stripping in ethtool with 'rx-vlan-stag-hw-parse', they shouldn't have any
> problem with understanding those function names.

Naming is hard.  My perspective is conveying the meaning without having
to be knowledgeable about ethtool VLAN commands.  My perspective is a
casual BPF-programmer that reads "bpf_xdp_metadata_rx_stag()".
Hopefully we can choose a name that says "vlan" somewhere, such that the
person reading this doesn't have to lookup and find the documentation to
deduct this code is related to VLANs.

> One possible improvement that comes to mind is maybe (similarly ethtool) calling
> c-tag just 'tag' and letting s-tag stay 'stag'. Because c-tag is this default
> 802.1q tag, which is supported by various hardware, while s-tag is significantly
> less widespread.
>
> But there are many options, really.
>
> What are your suggestions?

One suggestion is (the symmetrical):
 * bpf_xdp_metadata_rx_vlan_inner_tag
 * bpf_xdp_metadata_rx_vlan_outer_tag

As you say above the first "inner" VLAN tag is just the regular 802.1Q
VLAN tag.  The concept of C-tag and S-tag is from 802.1ad that
introduced the concept of double tagging.

Thus one could argue for shorter names like:
 * bpf_xdp_metadata_rx_vlan_tag
 * bpf_xdp_metadata_rx_vlan_outer_tag


> > > + * @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).
>
> Will specify that.
>
> > > + * 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.
> > > + *

(p.s. Googling I find multiple definitions of what the "S" in S-tag
means. The most reliable or statistically consistent seems to be
"Service tag", or "Service provider tag".)

The description for the renamed "bpf_xdp_metadata_rx_vlan_outer_tag"
should IMHO explain that the outer VLAN tag is often refered to as the 
S-tag (or Service-tag) in Q-in-Q (802.1ad) terminology.  Perhaps we can 
even spell out that some hardware support (and must be configured via 
ethtool) to extract this stag.

A dump of the tool rx-vlan related commands:

  $ ethtool -k i40e2 | grep rx-vlan
  rx-vlan-offload: on
  rx-vlan-filter: on [fixed]
  rx-vlan-stag-hw-parse: off [fixed]
  rx-vlan-stag-filter: off [fixed]




> > > + * 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.
>
> That's a good suggestion, I will expand the comment to describe error codes used
> so far.
>
> > 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;
> > > +}
> > > +