linux-kernel - Re: [PATCH bpf-next,v3 2/2] selftests/bpf: Enhance XDP Rx metadata handling

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Message-ID: <20250710102825.744f2653@kernel.org>
Date: Thu, 10 Jul 2025 10:28:25 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: "Song, Yoong Siang" <yoong.siang.song@...el.com>
Cc: Stanislav Fomichev <stfomichev@...il.com>, Daniel Borkmann
 <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, "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>, 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>, Magnus Karlsson
 <magnus.karlsson@...il.com>, Björn Töpel
 <bjorn@...nel.org>, "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
 Jonathan Lemon <jonathan.lemon@...il.com>, "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,v3 2/2] selftests/bpf: Enhance XDP Rx metadata
 handling

On Thu, 10 Jul 2025 15:35:32 +0000 Song, Yoong Siang wrote:
> Would it be advisable to update the documentation to indicate that
> drivers are expected to copy any device-reserved metadata from the
> metadata area? This would ensure that xdp_buff->data_meta is equal
> to xdp_buff->data before a BPF program is executed. This approach
> would allow BPF programs to freely manipulate the metadata area
> in XDP_REDIRECT scenarios.

Documenting sounds good.

> Additionally, I am uncertain about the need to overriding metadata in
> XDP_PASS scenarios. Should BPF programs refrain from overriding the
> metadata in this case?

IIRC XDP_PASS was the initial use case for the metadata area.
The driver needs to evacuate any HW metadata before handing over
to the XDP program.