I have a problem about vm connectivity using GRE tunnel and my environment is CentOS6.4 and OpenStack Havana. It is very slow between vm connectivity, but not between vm and internet.
ethtool -K eth0 gro off
But it is only helpful to vm and internet. And I also try to edit l3_agent.ini and dhcp_agent.ini to set the ovs_use_veth=true, but it is unuseful for me.