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

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Message-ID: <IA3PR11MB92545C3B2CD5778EE24244C1D84EA@IA3PR11MB9254.namprd11.prod.outlook.com>
Date: Tue, 8 Jul 2025 02:06:11 +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 9:45 AM, Jakub Kicinski <kuba@...nel.org> wrote:
>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.

I said so because I thought need to put back the timestamp
as prepend and then point skb_shared_hwtstamps.netdev_data to it
to support the ndo_get_tstamp().

I haven't study the code flow in detail, so I might be missing something.