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Re: [Xen-devel] [PATCH] Network Checksum Removal

On Wednesday 25 May 2005 04:14 pm, Keir Fraser wrote:
> On 25 May 2005, at 21:06, Ian Pratt wrote:
> >> There is a patch on netdev which can decrease the CPU load of
> >> bridging.
> >> specifically, it allows the bridge device to take advantage
> >> of the network device features (like hardware checksum
> >> offload).  Stephen Hemminger says it should go in the 2.6.13 kernel.
> >
> > Please can you post it as a patch so that we can include it in our
> > 2.6.11 patches directory.
> >
> > With the patch, csum offload will be much more interesting in the rx
> > case
> The code we already have offloads rx csums both for dom0 and domU's
> (the dom0 traffic has to be received through veth0 though, not the
> bridge device itself).

The problem with the bridge device is that all traffic generated in dom0 will 
be software checksummed, regardless of whether it needs to be or not.  In 
Xen's case, it will software checksum all traffic to domU, even though the 
vif device is advertising NETIF_F_NO_CSUM.  

This is because the stack doesn't see the features of the children devices of 
the bridge, only the features of the bridge device itself.  I created a quick 
hack to work around this, and started the discussion on the Linux netdev 
mailing list about how to fix the problem.  From this discussion, a patch was 
created which does most of what we want, but needs to be slightly modified to 
be optimal for Xen.  I will post the Xen optimized patch as soon as I have it 


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