linux-kernel - Re: [PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata Handling

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Message-ID: <20250707184449.42736a0a@kernel.org>
Date: Mon, 7 Jul 2025 18:44:49 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: "Song, Yoong Siang" <yoong.siang.song@...el.com>
Cc: "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Simon Horman
 <horms@...nel.org>, Jonathan Corbet <corbet@....net>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, "Jesper Dangaard
 Brouer" <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>,
 Stanislav Fomichev <sdf@...ichev.me>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>, Yonghong Song
 <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, Hao Luo
 <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko
 <mykolal@...com>, Shuah Khan <shuah@...nel.org>, "netdev@...r.kernel.org"
 <netdev@...r.kernel.org>, "linux-doc@...r.kernel.org"
 <linux-doc@...r.kernel.org>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "bpf@...r.kernel.org"
 <bpf@...r.kernel.org>, "linux-kselftest@...r.kernel.org"
 <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata
 Handling

On Tue, 8 Jul 2025 01:34:13 +0000 Song, Yoong Siang wrote:
> >For normal XDP my understanding is that its the driver's responsibility
> >to move the "reserved" stuff out of place before presenting the frame to
> >program.  
> 
> Is it means that driver needs to move out the "reserved" stuff before XDP program
> and then move back the stuff after XDP program for certain situation, like XDP_PASS?

Why would the driver need to move it back?
On XDP_PASS an skb is constructed, so the metadata should 
be transferred to the skb. There is no need to copy it back
as a prepend.