netdev - Re: xdp generic default option

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Date:   Wed, 19 Aug 2020 13:57:51 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: xdp generic default option

On Wed, Aug 19, 2020 at 2:29 AM Lorenzo Bianconi <lorenzo@...nel.org> wrote:
>
> Hi Andrii,
>
> working on xdp multi-buff I figured out now xdp generic is the default choice
> if not specified by userspace. In particular after commit 7f0a838254bd
> ("bpf, xdp: Maintain info on attached XDP BPF programs in net_device"), running
> the command below, XDP will run in generic mode even if the underlay driver
> support XDP in native mode:
>
> $ip link set dev eth0 xdp obj prog.o
> $ip link show dev eth0
> 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 xdpgeneric qdisc mq state UP mode DEFAULT
>    group default qlen 1024
>    link/ether f0:ad:4e:09:6b:57 brd ff:ff:ff:ff:ff:ff
>    prog/xdp id 1 tag 3b185187f1855c4c jited
>
> Is it better to use xdpdrv as default choice if not specified by userspace?
> doing something like:
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index a00aa737ce29..1f85880ee412 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -8747,9 +8747,9 @@ static enum bpf_xdp_mode dev_xdp_mode(u32 flags)
>  {
>         if (flags & XDP_FLAGS_HW_MODE)
>                 return XDP_MODE_HW;
> -       if (flags & XDP_FLAGS_DRV_MODE)
> -               return XDP_MODE_DRV;
> -       return XDP_MODE_SKB;
> +       if (flags & XDP_FLAGS_SKB_MODE)
> +               return XDP_MODE_SKB;
> +       return XDP_MODE_DRV;
>  }
>

I think the better way would be to choose XDP_MODE_DRV if ndo_bpf !=
NULL and XDP_MODE_SKB otherwise. That seems to be matching original
behavior, no?

It was not my intent to change the behavior, sorry about that. I'll
post patch a bit later today.


>  static bpf_op_t dev_xdp_bpf_op(struct net_device *dev, enum bpf_xdp_mode mode)
>
> Regards,
> Lorenzo