Source: Advanced Micro Devices
Virtualization enables live migration, the ability to move a running guest from one physical machine to another with little or no disruption to the guest or its users. Live migration allows various load-balancing and high-availability techniques to be implemented by the hypervisor and datacenter management software. Unfortunately for live migration, CPU features are added over time and existing guest Operating Systems (OSes) and applications are not well-equipped to handle CPU changes while the OSes are running. A system administrator would prefer to view a homogenous data center in which every machine can run every guest, but most real datacenters have several generations of machines that have been added over time.