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

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Message-ID: <066a0c0a-ad48-517a-4bd0-8920bdbf0dd8@iogearbox.net>
Date:   Fri, 28 May 2021 13:29:53 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        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>,
        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 12:54 PM, Toke Høiland-Jørgensen wrote:
> Daniel Borkmann <daniel@...earbox.net> writes:
>> 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.
> 
> We discussed this when creating xdpdump and decided to keep it as a
> separate tool for the time being. I forget the details of the
> discussion, maybe Eelco remembers.
> 
> Anyway, xdpdump does have a "pipe pcap to stdout" feature so you can do
> `xdpdump | tcpdump` and get the interactive output; and it will also
> save pcap information to disk, of course (using pcap-ng so it can also
> save metadata like XDP program name and return code).

Right, and this should yield a significantly better performance compared to
cloning & pushing traffic into AF_PACKET. I presume not many folks are aware
of xdpdump (yet) which is probably why such patch was created here.. a native
libpcap implementation could solve that aspect fwiw and additionally hook at
the same points as AF_PACKET via BPF but without the hassle/overhead of things
like dev_queue_xmit_nit() in fast path. (Maybe another option could be to have
a drop-in replacement libpcap.so for tcpdump using it transparently.)

Thanks,
Daniel