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

[Anthony Liguori <anthony@codemonkey.ws>]

On 07/16/2010 10:41 AM, Markus Armbruster wrote:
> Anthony Liguori<anthony@codemonkey.ws>  writes:
>
>    
>> On 07/15/2010 03:22 PM, 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.
>>>
>>> I opened the dumped files in wireshark and they looked fine. When using slirp
>>> all requests were dumped fine too.
>>>
>>>        
>> A less invasive way to do this would be to chain netdev devices.
>>
>> Basically:
>>
>> -netdev tap,fd=X,id=foo
>> -netdev dump,file=foo.pcap,netdev=foo,id=bar
>> -net nic,model=virtio,netdev=bar
>>      
> Is this really less invasive?  It breaks the simple 1:1 relationship
> between NIC and network backend.  All the code dealing with
> VLANClientState member peer needs to be touched.  For instance, this is
> the code to connect peers, in qemu_new_net_client():
>
>          if (peer) {
>              assert(!peer->peer);
>              vc->peer = peer;
>              peer->peer = vc;
>          }
>    

The peering code should all disappear.  I thought that's the whole point 
of this exercise?

I think the main advantage is we avoid adding special logic to handle 
dumping.  If we never have a case like this again, then perhaps it 
doesn't matter.

> Possibly worth it if we had a number of different things we want to
> insert between the end points, but I don't see that right now.
>
>    
>> I think this has some clear advantages to this architecturally.  From
>> a user perspective, the only loss is that you have to add the dump
>> device at startup (you can still enable/disable capture dynamically).
>>      
> I don't like this restriction at all.
>    

I don't either but I don't think it's a deal breaker.  I'm really open 
to either approach but I just wanted to make sure this one was considered.

Regards,

Anthony Liguori