netdev - Re: [PATCH] rtnetlink: add needed

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Message-ID: <8ce0399b-f9e0-4606-9b1b-ea1317d11a5d@iogearbox.net>
Date: Mon, 15 Sep 2025 20:30:20 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Stephen Hemminger <stephen@...workplumber.org>
Cc: Alasdair McWilliam <alasdair@...illiam.dev>, netdev@...r.kernel.org,
 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>
Subject: Re: [PATCH] rtnetlink: add needed_{head,tail}room attributes

On 9/15/25 7:20 PM, Stephen Hemminger wrote:
> On Mon, 15 Sep 2025 18:49:55 +0200
> Daniel Borkmann <daniel@...earbox.net> wrote:
>> On 9/15/25 6:32 PM, Alasdair McWilliam wrote:
>>> Various network interface types make use of needed_{head,tail}room values
>>> to efficiently reserve buffer space for additional encapsulation headers,
>>> such as VXLAN, Geneve, IPSec, etc. However, it is not currently possible
>>> to query these values in a generic way.
>>>
>>> Introduce ability to query the needed_{head,tail}room values of a network
>>> device via rtnetlink, such that applications that may wish to use these
>>> values can do so.
>>>
>>> Signed-off-by: Alasdair McWilliam <alasdair@...illiam.dev>
> 
> Why is this info needed in user space?
> How would an application use this?
> Seems like a hardware specific optimization that should be hidden in driver.

We would like to use this in Cilium. Basically our Cilium agent iterates over
the present devices based on the Cilium user config (e.g. direct routing/vxlan/
geneve/wireguard/etc) to then configure netkit in order to expose the
needed_{head,tail}room into K8s Pods, see also b9ed315d3c4c ("netkit: Allow
for configuring needed_{head,tail}room").

Thanks,
Daniel