netdev - Re: [PATCH bpf-next v2 1/6] net: tun: enable XDP metadata support

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Message-ID: <CACGkMEu0amsUNZS_EoJc40B=av90OJkpivDw3vCWwYJYAB68kA@mail.gmail.com>
Date: Wed, 19 Feb 2025 11:17:40 +0800
From: Jason Wang <jasowang@...hat.com>
To: Marcus Wichelmann <marcus.wichelmann@...zner-cloud.de>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, willemdebruijn.kernel@...il.com, 
	andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com, 
	kuba@...nel.org, pabeni@...hat.com, ast@...nel.org, daniel@...earbox.net, 
	andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org, 
	sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, mykolal@...com, 
	shuah@...nel.org, hawk@...nel.org
Subject: Re: [PATCH bpf-next v2 1/6] net: tun: enable XDP metadata support

On Tue, Feb 18, 2025 at 1:23 AM Marcus Wichelmann
<marcus.wichelmann@...zner-cloud.de> wrote:
>
> Enable the support for the bpf_xdp_adjust_meta helper function for XDP
> buffers initialized by the tun driver. This allows to reserve a metadata
> area that is useful to pass any information from one XDP program to
> another one, for example when using tail-calls.
>
> Whether this helper function can be used in an XDP program depends on
> how the xdp_buff was initialized. Most net drivers initialize the
> xdp_buff in a way, that allows bpf_xdp_adjust_meta to be used. In case
> of the tun driver, this is currently not the case.
>
> There are two code paths in the tun driver that lead to a
> bpf_prog_run_xdp and where metadata support should be enabled:
>
> 1. tun_build_skb, which is called by tun_get_user and is used when
>    writing packets from userspace into the device. In this case, the
>    xdp_buff created in tun_build_skb has no support for
>    bpf_xdp_adjust_meta and calls of that helper function result in
>    ENOTSUPP.
>
>    For this code path, it's sufficient to set the meta_valid argument of
>    the xdp_prepare_buff call. The reserved headroom is large enough
>    already.
>
> 2. tun_xdp_one, which is called by tun_sendmsg which again is called by
>    other drivers (e.g. vhost_net). When the TUN_MSG_PTR mode is used,
>    another driver may pass a batch of xdp_buffs to the tun driver. In
>    this case, that other driver is the one initializing the xdp_buff.
>
>    See commit 043d222f93ab ("tuntap: accept an array of XDP buffs
>    through sendmsg()") for details.
>
>    For now, the vhost_net driver is the only one using TUN_MSG_PTR and
>    it already initializes the xdp_buffs with metadata support and
>    sufficient headroom. But the tun driver disables it again, so the
>    xdp_set_data_meta_invalid call has to be removed.
>
> Signed-off-by: Marcus Wichelmann <marcus.wichelmann@...zner-cloud.de>
> ---

Acked-by: Jason Wang <jasowang@...hat.com>

Thanks