Re: [PATCH bpf-next v3 1/6] net: tun: enable XDP metadata support

[Jason Wang <jasowang@redhat.com>]

On Mon, Feb 24, 2025 at 11:29 PM Marcus Wichelmann
<marcus.wichelmann@hetzner-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@hetzner-cloud.de>

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

Thanks