linux-kernel - Re: [PATCH net-next v3] xdp: Add helpers for head length, headroom, and metadata length

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Message-ID: <e4cf6912-74fb-441f-ad05-82ea99d81020@kernel.org>
Date: Wed, 7 May 2025 21:04:51 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Jon Kohler <jon@...anix.com>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Zvi Effron <zeffron@...tgames.com>,
 Stanislav Fomichev <stfomichev@...il.com>, Jason Wang <jasowang@...hat.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 John Fastabend <john.fastabend@...il.com>, Simon Horman <horms@...nel.org>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
 Jacob Keller <jacob.e.keller@...el.com>
Subject: Re: [PATCH net-next v3] xdp: Add helpers for head length, headroom,
 and metadata length

On 07/05/2025 19.47, Jon Kohler wrote:
> 
> 
>> On May 7, 2025, at 1:21 PM, Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
>>
>>
>> Jesper Dangaard Brouer wrote:
>>>
>>>
>>> On 07/05/2025 19.02, Zvi Effron wrote:
>>>> On Wed, May 7, 2025 at 9:37 AM Jesper Dangaard Brouer <hawk@...nel.org> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 07/05/2025 15.29, Willem de Bruijn wrote:
>>>>>> Stanislav Fomichev wrote:
>>>>>>> On 05/06, Jon Kohler wrote:
>>>>>>>> Introduce new XDP helpers:
>>>>>>>> - xdp_headlen: Similar to skb_headlen
>>>>>
>>>>> I really dislike xdp_headlen(). This "headlen" originates from an SKB
>>>>> implementation detail, that I don't think we should carry over into XDP
>>>>> land.
>>>>> We need to come up with something that isn't easily mis-read as the
>>>>> header-length.
>>>>
>>>> ... snip ...
>>>>
>>>>>>> + * xdp_headlen - Calculate the length of the data in an XDP buffer
>>>>
>>>> How about xdp_datalen()?
>>>
>>> Yes, I like xdp_datalen() :-)
>>
>> This is confusing in that it is the inverse of skb->data_len:
>> which is exactly the part of the data not in the skb head.
>>
>> There is value in consistent naming. I've never confused headlen
>> with header len.
>>
>> But if diverging, at least let's choose something not
>> associated with skbs with a different meaning.
> 
> Brainstorming a few options:
> - xdp_head_datalen() ?
> - xdp_base_datalen() ?
> - xdp_base_headlen() ?
> - xdp_buff_datalen() ?
> - xdp_buff_headlen() ?
> - xdp_datalen() ? (ZivE, JesperB)
> - xdp_headlen() ? (WillemB, JonK, StanislavF, JacobK, DanielB)
> 

What about keeping it really simple: xdp_buff_len() ?

Or even simpler: xdp_len() as the function documentation already
describe this doesn't include frags.

To Jon, you seems to be on a cleanup spree:
For SKBs netstack have this diagram documented [1].  Which also explains
the concept of a "head" buffer, which isn't a concept for XDP.  I would
really like to see a diagram documenting both xdp_buff and xdp_frame
data structures via ascii art, like the one for SKBs. (Hint, this is
actually defined in the header file include/linux/skbuff.h, but
converted to RST/HTML format.)

[1] https://docs.kernel.org/networking/skbuff.html

--Jesper