The ethtool -c command returns "operation not supported" on ixgbevf VMs

Issue

• When testing with iperf3 we are experiencing performance issues with ixgbevf driver 2.7.12-k on virtual machine, too many retries and unable to reach expected 10G throughput.

• We have tried the same test using the Intel 2.14.2 driver which performed as expected with 9+G throughput and 0 retries.

• Observed differences:
  When running 2.7.12-k, I am unable to run ethtool -c eth1 to modify coalesce parameters, it throws the error message "Cannot get device coalesce settings: Operation not supported". The same ethtool -c eth1 works with 2.14.2

• There is only one IRQ active for TxRx pair with 2.7.12-k,
  29: 0 37643 37645 37635 37650 37647 37636 37656 PCI-MSI-edge eth1-TxRx-0
  30: 0 2 2 2 1 2 2 3 PCI-MSI-edge eth1

With 2.14.2 I see 2 active IRQ's balancing quite efficiently.

29: 0 14699 15011 14061 14240 13936 14399 13668 PCI-MSI-edge eth1-TxRx-0
30: 0 30715 30400 31336 31173 31474 31018 31746 PCI-MSI-edge eth1-TxRx-1
31: 0 2 5 5 4 2 2 2 PCI-MSI-edge eth1