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

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Message-ID: <2121D2EF-E554-4DCB-BB6A-93FB3975B064@nutanix.com>
Date: Thu, 8 May 2025 03:19:42 +0000
From: Jon Kohler <jon@...anix.com>
To: Jakub Kicinski <kuba@...nel.org>
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 May 7, 2025, at 8:18 PM, Jakub Kicinski <kuba@...nel.org> wrote:
> 
> On Wed, 7 May 2025 22:58:33 +0200 Jesper Dangaard Brouer wrote:
>>> There is a neat hint from Lorenzo’s change in bpf.h for bpf_xdp_get_buff_len()
>>> that talks about both linear and paged length. Also, xdp_buff_flags’s
>>> XDP_FLAGS_HAS_FRAGS says non-linear xdp buff.
>>> 
>>> Taking those hints, what about:
>>> xdp_linear_len() == xdp->data_end - xdp->data
>>> xdp_paged_len() == sinfo->xdp_frags_size
>>> xdp_get_buff_len() == xdp_linear_len() + xdp_paged_len()
>> 
>> 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