Future generations of AMD processors will not include 3DNow! CPU instructions. EVC mode is updated accordingly to hide this feature from older generation’s processors. So hosts which have 3DNow! Instructions will not be compatible with future generations AMD processors unless EVC mode is enabled. As future generation AMD processors will not have 3DNOW CPU instructions. So EVC mode must be enabled for vMotion compatibility between 3DNow! And Not 3DNow! CPU instructions. All AMD Opteron generations 3 are no 3DNow. Generations1 and 2 cannot be made vMotion compatible with hosts that have 3DNow! Instructions
CPU compatibility masks allows per virtual machine customization of CPU features visible to virtual machine
vCenter compares CPU features of Virtual machine with that of CPU features of the destination host to determine whether to allow or disallow migrations. In cases where you have to decide whether to maintain vMotion compatibility or enhanced CPU feature which guest OS might need, VMware provides check-box to configure individual Virtual Machine.You can configure more advance features as well at the bit level.