> 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
Doens't currently work on egress, but if/when we get a proper TX hook
that should be doable as well.
Wiring up XSK to AF_PACKET sounds a bit nonsensical: XSK is already a
transport to userspace, why would you need a second one?
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.
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).