[Qemu-devel] Re: [PATCH RFC 0/4] Dumping traffic when using netdev devices

[Jan Kiszka <jan.kiszka@web.de>]

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Miguel Di Ciurcio Filho wrote:
> Hello,
> 
> This is a prototype suggestion. I mostly copied and pasted the code from
> net/dump.c into net.c and made some adjustments. There is no command line
> parsing involved yet, just the internals and small changes in net/tap.c and
> net/slirp.c do make the thing work.
> 
> In my tests, using tap as backend, e1000 as a guest device and running iperf from
> guest to host, the overhead of dumping the traffic caused a loss of around 30%
> of performance.

No surprise, dump writes are synchronous. The day this is actually
hurting someone, we may see asynchronous patches.

> 
> I opened the dumped files in wireshark and they looked fine. When using slirp
> all requests were dumped fine too.
> 
> Bugs/limitations:
> - I have no clue on how to deal with tap+vhost, is it necessary?

I don't think so. Do you can still do "tcpdump -i tap0" in that
scenario, don't you? Then definitely not.

> - When using virtio-net, I'm not sure how to handle iovec when vnet_hdr=on

Let the sending peer report (offset field or callback) where to find the
payload in a frame.

That channel - or a separate one - could also be used to detect if a
peer supports dumping at all (vhost...). Then no peer code need to be
extended with dump management code, all could be moved into net.c

> - Create a function to add dump to a netdev on the fly, is it necessary?

Yes. That way you don't need to shutdown&restart your VM just to enable
or disable dumping - quite important in certain tests.

> - Handle cleanups
> 
> Miguel Di Ciurcio Filho (4):
>   net/dump: Make pcap structures public
>   net: Introduce NetClientDump and auxiliary functions
>   net/tap: Suggested support for NetClientDump
>   net/slirp: Suggested support for NetClientDump
> 
>  net.c         |   90 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  net.h         |    8 +++++
>  net/dump.c    |   21 -------------
>  net/dump.h    |   21 +++++++++++++
>  net/slirp.c   |    3 ++
>  net/tap.c     |    3 ++
>  qemu-common.h |    1 +
>  7 files changed, 126 insertions(+), 21 deletions(-)
> 

Thanks for picking this up!

Jan


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