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

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Message-ID: <20250508182857.2b60301e@kernel.org>
Date: Thu, 8 May 2025 18:28:57 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Jon Kohler <jon@...anix.com>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>, Willem de Bruijn
 <willemdebruijn.kernel@...il.com>, 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>, 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 Thu, 8 May 2025 03:19:42 +0000 Jon Kohler wrote:
> >> I like xdp_linear_len() as it is descriptive/clear.  
> > 
> > FWIW I don't feel strongly but my very weak preference would be 
> > not to merge this. I already know I'll be looking at the definitions
> > every time. Is it obvious to everyone in this thread whether "headroom"
> > includes the metadata length? It's not obvious to me. But the patch
> > seems quite popular so 🤷️  
> 
> Jespers suggestion to have a DOC: on this hopefully will be helpful.
> 
> I’ll try my hand that that and see what sort of trouble I can get into

I hate to dwell since I already said I don't feel strongly :) but the
doc is orthogonal, it helps people who don't understand the geometry
to understand it. My concern was that I understand the geometry, but
now I will also need to remember what we decided to call "headroom"
since (unlike skbs) XDP has all sort of stuff packed in front of the
packet data :)