netdev - Re: [PATCH bpf-next] xsk: support AF

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Date:   Fri, 28 May 2021 12:22:40 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Magnus Karlsson <magnus.karlsson@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Xie He <xie.he.0141@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        John Ogness <john.ogness@...utronix.de>,
        Wang Hai <wanghai38@...wei.com>,
        Tanner Love <tannerlove@...gle.com>,
        Eyal Birger <eyal.birger@...il.com>,
        Menglong Dong <dong.menglong@....com.cn>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next] xsk: support AF_PACKET

On 5/28/21 12:00 PM, Magnus Karlsson wrote:
> On Fri, May 28, 2021 at 11:52 AM Jesper Dangaard Brouer
> <brouer@...hat.com> wrote:
>> On Fri, 28 May 2021 17:02:01 +0800
>> Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>>> On Fri, 28 May 2021 10:55:58 +0200, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>>> Xuan Zhuo <xuanzhuo@...ux.alibaba.com> writes:
>>>>
>>>>> In xsk mode, users cannot use AF_PACKET(tcpdump) to observe the current
>>>>> rx/tx data packets. This feature is very important in many cases. So
>>>>> this patch allows AF_PACKET to obtain xsk packages.
>>>>
>>>> You can use xdpdump to dump the packets from the XDP program before it
>>>> gets redirected into the XSK:
>>>> https://github.com/xdp-project/xdp-tools/tree/master/xdp-dump
>>>
>>> Wow, this is a good idea.
>>
>> Yes, it is rather cool (credit to Eelco).  Notice the extra info you
>> can capture from 'exit', like XDP return codes, if_index, rx_queue.
>>
>> The tool uses the perf ring-buffer to send/copy data to userspace.
>> This is actually surprisingly fast, but I still think AF_XDP will be
>> faster (but it usually 'steals' the packet).
>>
>> Another (crazy?) idea is to extend this (and xdpdump), is to leverage
>> Hangbin's recent XDP_REDIRECT extension e624d4ed4aa8 ("xdp: Extend
>> xdp_redirect_map with broadcast support").  We now have a
>> xdp_redirect_map flag BPF_F_BROADCAST, what if we create a
>> BPF_F_CLONE_PASS flag?
>>
>> The semantic meaning of BPF_F_CLONE_PASS flag is to copy/clone the
>> packet for the specified map target index (e.g AF_XDP map), but
>> afterwards it does like veth/cpumap and creates an SKB from the
>> xdp_frame (see __xdp_build_skb_from_frame()) and send to netstack.
>> (Feel free to kick me if this doesn't make any sense)
> 
> This would be a smooth way to implement clone support for AF_XDP. If
> we had this and someone added AF_XDP support to libpcap, we could both
> capture AF_XDP traffic with tcpdump (using this clone functionality in
> the XDP program) and speed up tcpdump for dumping traffic destined for
> regular sockets. Would that solve your use case Xuan? Note that I have
> not looked into the BPF_F_CLONE_PASS code, so do not know at this
> point what it would take to support this for XSKMAPs.

Recently also ended up with something similar for our XDP LB to record pcaps [0] ;)
My question is.. tcpdump doesn't really care where the packet data comes from,
so why not extending libpcap's Linux-related internals to either capture from
perf RB or BPF ringbuf rather than AF_PACKET sockets? Cloning is slow, and if
you need to end up creating an skb which is then cloned once again inside AF_PACKET
it's even worse. Just relying and reading out, say, perf RB you don't need any
clones at all.

Thanks,
Daniel

   [0] https://cilium.io/blog/2021/05/20/cilium-110#pcap