Another interesting problem… a client was trying to add a new ESXi host into an existing cluster and was getting an error that the EVC mode of the cluster didn’t match the CPU capabilities and was generating this error message;
This wasn’t just the normal CPU compatibility issue… as the Dell M620 server that we were trying to add to the cluster was the exact same model and CPU as the current ESXi hosts that were in the cluster.
I eventually discovered that the existing ESXi hosts were running 6.5 Update 2 where as the host we were trying to add to the cluster was running 6.5 Update 1. It turns out that there were come CPU microcode updates in 6.5 Update 2 that were intended to mitigate the Spectre v2 vulnerability and those was tripping up the Enhanced vMotion Compatibility check between the ESXi hosts.
I found the following VMware knowledge base article that details the issue;
https://kb.vmware.com/s/article/52085
I upgraded the new ESXi server to 6.5 Update 2 and was able to add it to the cluster without issues.
Cheers!