netdev - Re: [PATCH bpf] xdp: Fix spurious packet loss in generic XDP TX path

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Date:   Fri, 1 Jul 2022 16:58:15 +0200
From:   Johan Almbladh <johan.almbladh@...finetworks.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        John Fastabend <john.fastabend@...il.com>, song@...nel.org,
        martin.lau@...ux.dev, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, jolsa@...nel.org,
        bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] xdp: Fix spurious packet loss in generic XDP TX path

On Fri, Jul 1, 2022 at 3:29 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 7/1/22 11:57 AM, Eric Dumazet wrote:
> > On Fri, Jul 1, 2022 at 11:43 AM Johan Almbladh
> > <johan.almbladh@...finetworks.com> wrote:
> >>
> >> The byte queue limits (BQL) mechanism is intended to move queuing from
> >> the driver to the network stack in order to reduce latency caused by
> >> excessive queuing in hardware. However, when transmitting or redirecting
> >> a packet with XDP, the qdisc layer is bypassed and there are no
> >> additional queues. Since netif_xmit_stopped() also takes BQL limits into
> >> account, but without having any alternative queuing, packets are
> >> silently dropped.
> >>
> >> This patch modifies the drop condition to only consider cases when the
> >> driver itself cannot accept any more packets. This is analogous to the
> >> condition in __dev_direct_xmit(). Dropped packets are also counted on
> >> the device.
> >
> > This means XDP packets are able to starve other packets going through a qdisc,
> > DDOS attacks will be more effective.
> >
> > in-driver-XDP use dedicated TX queues, so they do not have this
> > starvation issue.
> >
> > This should be mentioned somewhere I guess.
>
> +1, Johan, could you add this as comment and into commit description in a v2
> of your fix? Definitely should be clarified that it's limited to generic XDP.

Thanks for the review.

Daniel, I will prepare a v2 shortly.

Thanks,
Johan