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

[Larysa Zaremba <larysa.zaremba@intel.com>]

On Tue, Jul 04, 2023 at 12:23:45PM +0200, Jesper Dangaard Brouer wrote:
> 
> 
> On 04/07/2023 10.23, Larysa Zaremba wrote:
> > 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 <larysa.zaremba@intel.com>
> > > > ---
> > > >   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/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.
> > 
> 
> Do we really care about the "EtherType" proto (in VLAN speak TPID = Tag
> Protocol IDentifier)?
> I mean, it can basically only have two values[1], and we just wanted to
> know if it is a VLAN (that hardware offloaded/removed for us):

If we assume everyone follows the standard, this would be correct.
But apparently, some applications use some ambiguous value as a TPID [0].

So it is not hard to imagine, some NICs could alllow you to configure your 
custom TPID. I am not sure if any in-tree drivers actually do this, but I think 
it's nice to provide some flexibility on XDP level, especially considering 
network stack stores full vlan_proto.

[0] 
https://techhub.hpe.com/eginfolib/networking/docs/switches/7500/5200-1938a_l2-lan_cg/content/495503472.htm

> 
>  static __always_inline int proto_is_vlan(__u16 h_proto)
>  {
> 	return !!(h_proto == bpf_htons(ETH_P_8021Q) ||
> 		  h_proto == bpf_htons(ETH_P_8021AD));
>  }
> 
> [1] https://github.com/xdp-project/bpf-examples/blob/master/include/xdp/parsing_helpers.h#L75-L79
> 
> Cc. Andrew Lunn, as I notice DSA have a fake VLAN define ETH_P_DSA_8021Q
> (in file include/uapi/linux/if_ether.h)
> Is this actually in use?
> Maybe some hardware can "VLAN" offload this?
> 
> 
> > 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).
> > 
> 
> Hmm, I think we can improve this further. This text becomes part of the
> documentation for end-users (target audience).  Thus, I think it is
> worth being more verbose and even mention the existing defines that we
> are expecting end-users to take advantage of.
> 
> What about:
> 
> In case of success. The VLAN EtherType is stored in vlan_proto (usually
> either ETH_P_8021Q or ETH_P_8021AD) also known as TPID (Tag Protocol
> IDentifier). The VLAN tag is stored in vlan_tag, which is a 16-bit field
> containing sub-fields (PCP+DEI+VID). The VLAN ID (VID) is 12-bits
> commonly extracted using mask VLAN_VID_MASK (0x0fff).  For the meaning
> of the sub-fields Priority Code Point (PCP) and Drop Eligible Indicator
> (DEI) (formerly CFI) please reference other documentation. Remember
> these 16-bit fields are stored in network-byte. Thus, transformation
> with byte-order helper functions like bpf_ntohs() are needed.
> 

AFAIK, vlan_tag is stored in host byte order, this is how it is in skb.
In ice, we receive VLAN tag in descriptor already in LE.
Only protocol is BE (network byte order). So I would replace the last 2 
sentences with the following:

vlan_tag is stored in host byte order, so no byte order conversion is needed.
vlan_proto is stored in network byte order, the suggested way to use this value:

vlan_proto == bpf_htons(ETH_P_8021Q)

> 
> 
> > > 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;
> > > > +}
> > > > +
> 
>