Re: [Qemu-devel] [PATCH 4/5] net/dump: Add dump option for netdev devices

[Thomas Huth <thuth@redhat.com>]

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On Fri, 26 Jun 2015 10:44:59 +0100
Stefan Hajnoczi <stefanha@gmail.com> wrote:

> On Wed, Jun 24, 2015 at 05:56:20PM +0200, Thomas Huth wrote:
> > diff --git a/net/net.c b/net/net.c
> > index cc36c7b..8871b77 100644
> > --- a/net/net.c
> > +++ b/net/net.c
> > @@ -568,6 +568,12 @@ ssize_t qemu_deliver_packet(NetClientState *sender,
> >          return 0;
> >      }
> >  
> > +    if (nc->netdev_dump_enabled) {
> > +        net_dump_receive(nc, data, size);
> > +    } else if (sender->netdev_dump_enabled) {
> > +        net_dump_receive(sender, data, size);
> > +    }
...
> Perhaps dumping should happen after ->receive() has returned size.  If
> ->receive() returns -1 the packet is discarded, and if it returns 0 the
> packet is queued (not delivered yet).
> 
> If you dump unconditionally before ->receive() you will see queued
> packets dumped multiple times (each time the queue gets flushed).

I've now tried this, but then I suddenly get the packets in the wrong
order in the dump file (when using slirp networking), e.g.:

  1   0.000000     10.0.2.2 -> 255.255.255.255 DHCP 590 DHCP Offer
  2   0.000012      0.0.0.0 -> 255.255.255.255 DHCP 342 DHCP Discover

instead of:

  1   0.000000      0.0.0.0 -> 255.255.255.255 DHCP 342 DHCP Discover
  2   0.000035     10.0.2.2 -> 255.255.255.255 DHCP 590 DHCP Offer

Looks like with slirp, the answer is already sent before the initial
receive() function returns?

Any idea how to avoid that issue? If not, I think I'll simply keep the
dump hooks before calling the receive functions.

 Thomas

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