Ok we figured out how to set it to Ethernet mode in FreeBSD 10 (you need to have the mlxen kernel driver loaded):
sysctl sys.device.mlx4_core0.mlx4_port1=eth
We're testing using iperf between 2 servers and consistently getting 10gbps with about 5% CPU usage on a dual xeon which is pretty good.
Now my new question is:
Using IPoIB we've been able to hit 20+Gbps.
Is Ethernet mode locked to 10gbps or is it just an incorrect setting?
UPDATE: Looking at ifconfig the interface is registering as a 10Gbase-CX4. Not sure if it's a limit of the cable we're using or possible limitation of the adapter itself (ConnectX-3 based onboard adapter but listed as supporting QDR, not FDR)... anyone know?