OK good news here, the performance drop on the myri was caused by a
problem between the keyboard and the chair. After the reboot series,
I forgot to reload the firmware so the driver used the less efficient
firmware from the NIC (it performs just as if LRO is disabled).
That makes me think that I should try 3.8-rc2 since LRO was removed
The only remaining issue really is the loopback then.
Just for the record, I tested 3.8-rc2, and the myri works as fast with
GRO there as it used to work with LRO in previous kernels. The softirq
work has increased from 26 to 48% but there is no performance drop when
using GRO anymore. Andrew has done a good job !