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Re: [Xen-devel] Checksumming problem in pv_ops dom0 kernel / netback

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Checksumming problem in pv_ops dom0 kernel / netback
From: "S.H. Verbrugge" <ief@xxxxxxxxx>
Date: Thu, 18 Mar 2010 00:48:09 +0100
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On Thu, Mar 18, 2010 at 10:25:34AM +1100, James Harper wrote:
> > 
> > Yeah, I've seen that. It does not give that much information however.
> > 
> > More specifically, I've tested with two dom0 pv_ops kernels now, and
> since
> > it's still
> > reproducable on the latest xen/stable platform, I'm guessing it either
> has to
> > do with vlan'ing or
> > the tg3 driver , in combination with netback.
> > 
> > I was hoping some Xen developers could shed some light on this.
> > I already conversed with Jeremy about this, and he pointed me to this
> mailing
> > list.
> > 
> This may not be relavant, but I have seen problems with the following
> combination:
> br0:
>  eth0
>  <netback devices>
> br1:
>  eth0.2
>  <netback devices>
> Some (most?) network hardware cannot provide checksum/large send offload
> functions for packets that use vlan tagging, but Linux doesn't quite
> understand that and gets confused, so when such a packet comes off of
> netback and is sent to eth0.2, the LSO/checksum function should be
> performed in software but isn't.
> I haven't yet figured out of the problem is that the driver is
> incorrectly reporting that offload is supported on the vlan device or if
> the rest of Linux isn't taking the appropriate action...

Hmm, just to set the record straight, there's vlan'ing on the switch (containing
several ports in a single physical network), but no tagging as far as I know.

However, I've read some previous problems using checksum offloading in the TG3 
Perhaps the two are related (netback / tg3 checksums), but I have no way of 
determining that.

Isn't there some way of patching the netback driver, so it does not support 
Maybe I'm way off base here..

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