Re: [uml-devel] uml "ip header error" for large (fragmented) udp packets

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

On Wednesday 06 October 2004 23:48, Lars Ellenberg wrote:
> / 2004-10-06 22:35:39 +0200
>
> \ Andi Kleen:
> > On Wed, Oct 06, 2004 at 08:48:23PM +0200, Lars Ellenberg wrote:
> > > > So this could help you for now, while we try to find a clue about
> > > > this. Quite frankly, I must say that I'm not seeing any network
> > > > kernel hacker here (correct me if I'm wrong), so it will take some
> > > > time to debug it. Maybe Gerd Knorr is an exception, actually.
> > >
> > > Well, then I take Andi Kleen and Lars Marowsky-Br?e into CC for now.
> > > Lars, because I expect him to be interessted in having UML as full
> > > featured cluster simulation tool available, and Andi because I hope he
> > > might know the network code much better than me...
> > >
> > > FYI, full thread can be found for example at
> > > http://thread.gmane.org/gmane.linux.uml.devel/4607
> >
> > Paolo's analysis is basically correct. loopback sets this flag
> > for better performance.  Actually in 2.6 it probably doesn't help
> > very much anymore because TCP can do checksum copy RX now, and that
> > would get the checksum basically for free. But it's still there
> > and may still make things slightly faster.
> >
> > If UML taps the packets from lo it will see incorrect checksums.
It does not, so the solution is not the right one.
> > Using a tun or ethertap device would avoid this. In the worst
> > case you could also just delete the flag from the loopback
> > interface, it's only an optimization.
> >
> > -Andi
>
> unfortunately ethertap transport does not work either,
> at least if UML is 2.6.6 and host kernel is 2.4.21-suse-whatever...
> I did not try other combinations yet, but I doubt that changes a thing.
>
> you suggest that we remove NET_IF_F_NO_CSUM from lo in the host?
> ok, I'll try recompile my host then, and followup if that helps.

No, I think he spoke about the guest; also he misunderstood a bit the problem, 
since the packets do not go through the "lo" interface inside UML.

-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729


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