I have Ubuntu 14.04 (CIS benchmark image, but I don't think it's related). I installed ENA according to http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/enhanced-networking-ena.html#test-enhanced-networking-ena. I imaged and relaunched my AMI as m4.x16large and I do have ENA active:
ethtool -i eth0 driver: ena version: 1.5.0g
I ran update-initramfs (tried both '-u' and '-c'), created AMI, and tried to launch as c5.4xlarge. boot fails with:
Begin: Mounting root file system ... Begin: Running /scripts/local-top ... /scripts/local-top/cryptroot: line 24: dirname: not found done. Gave up waiting for root device. ......
Any ideas? TIA, Vitaly