Hi Rodrigo, there is a possibility that the problem is not a regression
in handling GRO but actually supporting GRO. I can find the following
commit in 3.13-rc1:

commit 99d3d587b2b4314ccc8ea066cb327dfb523d598e
Author: Wei Liu <[email protected]>
Date:   Mon Sep 30 13:46:34 2013 +0100

    xen-netfront: convert to GRO API

Now I tried to reproduce the performance issue locally and using a
Xen-4.4.1 Trusty host which runs a Trusty PV guest. On the guest side I
start iperf in server mode (since that is the receive side, but to be
sure I reversed the setup with the same results) and on a desktop
running Trusty, I start iperf in client mode connecting to the PV guest.
The desktop and host have 1Gbit NICs. With that I get an average of
about 850Mbit/sec over 10 runs. Which is as good as I would expect it.
And this does not change significantly whether I enable or disable GRO.

Now what we do not know is what gets involved network-wise between your
server and the guests. Not really sure how this would possibly happen.

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https://bugs.launchpad.net/bugs/1391339

Title:
  Trusty kernel inbound network performance regression when GRO is
  enabled

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