Suggestions - post the dmesg lines for the network interface (minus mac addr, if you are paranoid) from both GENERIC and XEN3_DOM0. It is possible that this issue is related to not only Xen but also the driver for the particular network interface - run iperf to localhost both ways. That exercises TCP and the user/kernel path for data, but not the physical network interface - For any network issues, I recommend doing "netstat -s" before and after a test, and then diffing the results, trying to understand the origin of all counter changes. For example in your case, there probably shouldn't be any dropped packets. So if that increments -- or more importantly some other counter none of us have thought about -- that's interesting. - Run iperf in UDP mode. This separates TCP's congestion reaction from ability to push packets. - This is hard, and probably not that useful in this case, but graphics/xplot-devel has a script tcpdump2xplot, which probably needs updating for tcpdump format drift, that is really useful for visualizing tcp behavior. But if netstat -s shows no drops, this isn't so useful here.
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