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

From: Jesper Dangaard Brouer
Date: Tue May 23 2023 - 06:17:42 EST



On 22/05/2023 17.48, Larysa Zaremba wrote:
On Mon, May 22, 2023 at 10:37:33AM +0200, Jesper Dangaard Brouer wrote:


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


AFAIK, outer tag is a broader term, it's pretty often used for stacked 802.1Q
headers. I can't find what exactly is an expected behavior for rxvlan and
rx-vlan-stag-hw-parse in ethtool, but iavf documentation states that rxvlan
"enables outer or single 802.1Q VLAN stripping" and rx-vlan-stag-hw-parse
"enables outer or single 802.1ad VLAN stripping". This is in consistent with how
ice hardware behaves. More credible sources would be welcome.


It would be good to figure out how other hardware behaves.

The iavf doc sounds like very similar behavior from both functions, just 802.1Q vs 802.1ad.
Sounds like both will just pop/strip the outer vlan tag.
I have seen Ethertype 802.1Q being used (in practice) for double tagged
packets, even-though 802.1ad should have been used to comply with the
standard.

What about:
* bpf_xdp_metadata_rx_vlan_tag
* bpf_xdp_metadata_rx_vlan_qinq_tag


This sounds good to me.

I do wonder if we really need two functions for this?
Would one function be enough?

Given the (iavf) description, the functions basically does the same.
Looking at your ice driver implementation, they could be merged into one
function, as it is the same location in the descriptor.



+ * @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]

[...]