xennet vs Windows checksum offload - help wanted

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

There's a long-established problem with xennet when speaking to a Windows 
guest on the same host (with XenServer/Citrix Hypervisor/XCP at least).

If both the NetBSD and Windows domUs are on the same host, the network 
keeps stalling and performance is horrible. If they are on separate hosts, 
everything is as it should be. Similarly, going to the Advanced settings 
on the NIC within Windows and disabling TCP Checksum offload will resolve 
the performance problems.

i.e. if the network traffic doesn't hit a physical NIC and is bridged 
within the host (yes, I know that XenServer uses openvswitch, not 
bridging by default now), checksums must be wrong when they get to 
Windows.

I can offer test VMs if anyone would like to work with me to fix this.

--
Stephen