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

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Message-ID: <IA3PR11MB9254DC4B7984E014206A1FBFD84EA@IA3PR11MB9254.namprd11.prod.outlook.com>
Date: Tue, 8 Jul 2025 01:34:13 +0000
From: "Song, Yoong Siang" <yoong.siang.song@...el.com>
To: Jakub Kicinski <kuba@...nel.org>
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 Tuesday, July 8, 2025 4:55 AM, Jakub Kicinski <kuba@...nel.org> wrote:
>On Tue,  1 Jul 2025 12:29:38 +0800 Song Yoong Siang wrote:
>>                         |<---sizeof(xdp_meta)--|
>>                         |                      |
>>                  struct xdp_meta               rx_desc->address
>>                         ^                      ^
>>                         |                      |
>> +----------+----------------------+------------+------+
>> | headroom |    custom metadata   |  reserved  | data |
>> +----------+----------------------+------------+------+
>>            ^                      ^            ^
>>            |                      |            |
>>     struct xdp_meta     xdp_buff->data_meta    xdp_buff->data
>>            |                      |
>>            |<---sizeof(xdp_meta)--|
>
>Huh. Did AF_XDP maintainers explicitly sign off on this or it's just how
>IGC implementation works and nobody noticed?
>
Previously,  IGC do copy out the Rx hwts from metadata area,
so no problem when implementing XDP Rx metadata.

After that, net_device_ops.ndo_get_tstamp() is added into IGC to
support timestamping from both free-running clock and adjustable clock.
The 2 timers are stored in the metadata area, thus causing the issue.

>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?

IMHO, if driver is allowed to use some portion of the metadata area, then
the packet processing will be more efficiency and also align with the "zero-copy" idea.

Any thoughts?