Subject: Re: [PATCH bpf-next v2 09/20] xdp: Add VLAN tag hint

From: Larysa Zaremba <[email-masked]>
Date: Tue, 4 Jul 2023 10:23:05 +0200

On Mon, Jul 03, 2023 at 01:15:34PM -0700, John Fastabend wrote:
> Larysa Zaremba wrote:
> > Implement functionality that enables drivers to expose VLAN tag
> > to XDP code.
> > 
> > Signed-off-by: Larysa Zaremba <[email-masked]>
> > ---
> >  Documentation/networking/xdp-rx-metadata.rst |  8 +++++++-
> >  include/linux/netdevice.h                    |  2 ++
> >  include/net/xdp.h                            |  2 ++
> >  kernel/bpf/offload.c                         |  2 ++
> >  net/core/xdp.c                               | 20 ++++++++++++++++++++
> >  5 files changed, 33 insertions(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/networking/xdp-rx-metadata.rst b/Documentation/networking/xdp-rx-metadata.rst
> > index 25ce72af81c2..ea6dd79a21d3 100644
> > --- a/Documentation/networking/xdp-rx-metadata.rst
> > +++ b/Documentation/networking/xdp-rx-metadata.rst
> > @@ -18,7 +18,13 @@ Currently, the following kfuncs are supported. In the future, as more
> >  metadata is supported, this set will grow:
> >  
> >  .. kernel-doc:: net/core/xdp.c
> > -   :identifiers: bpf_xdp_metadata_rx_timestamp bpf_xdp_metadata_rx_hash
> > +   :identifiers: bpf_xdp_metadata_rx_timestamp
> > +
> > +.. kernel-doc:: net/core/xdp.c
> > +   :identifiers: bpf_xdp_metadata_rx_hash
> > +
> > +.. kernel-doc:: net/core/xdp.c
> > +   :identifiers: bpf_xdp_metadata_rx_vlan_tag
> >  
> >  An XDP program can use these kfuncs to read the metadata into stack
> >  variables for its own consumption. Or, to pass the metadata on to other
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index b828c7a75be2..4fa4380e6d89 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -1658,6 +1658,8 @@ struct xdp_metadata_ops {
> >  	int	(*xmo_rx_timestamp)(const struct xdp_md *ctx, u64 *timestamp);
> >  	int	(*xmo_rx_hash)(const struct xdp_md *ctx, u32 *hash,
> >  			       enum xdp_rss_hash_type *rss_type);
> > +	int	(*xmo_rx_vlan_tag)(const struct xdp_md *ctx, u16 *vlan_tag,
> > +				   __be16 *vlan_proto);
> >  };
> >  
> >  /**
> > diff --git a/include/net/xdp.h b/include/net/xdp.h
> > index 6381560efae2..89c58f56ffc6 100644
> > --- a/include/net/xdp.h
> > +++ b/include/net/xdp.h
> > @@ -389,6 +389,8 @@ void xdp_attachment_setup(struct xdp_attachment_info *info,
> >  			   bpf_xdp_metadata_rx_timestamp) \
> >  	XDP_METADATA_KFUNC(XDP_METADATA_KFUNC_RX_HASH, \
> >  			   bpf_xdp_metadata_rx_hash) \
> > +	XDP_METADATA_KFUNC(XDP_METADATA_KFUNC_RX_VLAN_TAG, \
> > +			   bpf_xdp_metadata_rx_vlan_tag) \
> >  
> >  enum {
> >  #define XDP_METADATA_KFUNC(name, _) name,
> > diff --git a/kernel/bpf/offload.c b/kernel/bpf/offload.c
> > index 8a26cd8814c1..986e7becfd42 100644
> > --- a/kernel/bpf/offload.c
> > +++ b/kernel/bpf/offload.c
> > @@ -848,6 +848,8 @@ void *bpf_dev_bound_resolve_kfunc(struct bpf_prog *prog, u32 func_id)
> >  		p = ops->xmo_rx_timestamp;
> >  	else if (func_id == bpf_xdp_metadata_kfunc_id(XDP_METADATA_KFUNC_RX_HASH))
> >  		p = ops->xmo_rx_hash;
> > +	else if (func_id == bpf_xdp_metadata_kfunc_id(XDP_METADATA_KFUNC_RX_VLAN_TAG))
> > +		p = ops->xmo_rx_vlan_tag;
> >  out:
> >  	up_read(&bpf_devs_lock);
> >  
> > diff --git a/net/core/xdp.c b/net/core/xdp.c
> > index 41e5ca8643ec..f6262c90e45f 100644
> > --- a/net/core/xdp.c
> > +++ b/net/core/xdp.c
> > @@ -738,6 +738,26 @@ __bpf_kfunc int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, u32 *hash,
> >  	return -EOPNOTSUPP;
> >  }
> >  
> > +/**
> > + * bpf_xdp_metadata_rx_vlan_tag - Get XDP packet outermost VLAN tag with protocol
> > + * @ctx: XDP context pointer.
> > + * @vlan_tag: Destination pointer for VLAN tag
> > + * @vlan_proto: Destination pointer for VLAN protocol identifier in network byte order.
> > + *
> > + * In case of success, vlan_tag contains VLAN tag, including 12 least significant bytes
> > + * containing VLAN ID, vlan_proto contains protocol identifier.
> 
> Above is a bit confusing to me at least.
> 
> The vlan tag would be both the 16bit TPID and 16bit TCI. What fields
> are to be included here? The VlanID or the full 16bit TCI meaning the
> PCP+DEI+VID?

It contains PCP+DEI+VID, in patch 16 ("selftests/bpf: Add flags and new hints to 
xdp_hw_metadata") this is more clear, because the tag is parsed.

What about rephrasing it this way:

In case of success, vlan_proto contains VLAN protocol identifier (TPID), 
vlan_tag contains the remaining 16 bits of a 802.1Q tag (PCP+DEI+VID).

> I think by "including 12 least significant bytes" you
> mean bits,

Yes, my bad.

> but also not clear about those 4 other bits.
> 
> I can likely figure it out in next patches from implementation but
> would be nice to clean up docs.
> 
> > + *
> > + * Return:
> > + * * Returns 0 on success or ``-errno`` on error.
> > + * * ``-EOPNOTSUPP`` : device driver doesn't implement kfunc
> > + * * ``-ENODATA``    : VLAN tag was not stripped or is not available
> > + */
> > +__bpf_kfunc int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx, u16 *vlan_tag,
> > +					     __be16 *vlan_proto)
> > +{
> > +	return -EOPNOTSUPP;
> > +}
> > +
> >  __diag_pop();
> >  
> >  BTF_SET8_START(xdp_metadata_kfunc_ids)
> > -- 
> > 2.41.0
> > 
> 
> 
>