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

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

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@intel.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?
> 
> 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;
>>> +}
>>> +
>>
>