It has been a while since I have had to enable EVC, but I needed to the other day in the office. I created a cluster with a HP DL380 G7 and an older HP DL380 G5. When I tried to turn EVC on for the cluster, I ran into this error. “The host cannot be admitted to the cluster’s current Enhanced vMotion Compatibility mode. Powered-on or suspended virtual machines on the host may be using CPU features hidden by that mode.” This message is telling you that the current machines that are powered on are using the technology from the newer processor and in order to turn on EVC for the cluster, the VMs need to be powered off. So…I powered off all of my VMs on the DL380 G7 (newer host).
Now depending on the processor generation, you have to change the EVC Mode. For mine, I chose the Intel “Penryn” Generation” and I still had a green check box. If your hosts don’t support the EVC mode, it will let you know in the Compatibility pane. The processor support documentation can be found here.