Every hypervisor has its own policy for what a guest virtual machine will see for its CPUs by default. Whereas some hypervisors decide which CPU host physical machine features will be available for the guest virtual machine, QEMU/KVM presents the guest virtual machine with a generic model named qemu32 or qemu64. These hypervisors perform more advanced filtering, classifying all physical CPUs into a handful of groups and have one baseline CPU model for each group that is presented to the guest virtual machine. Such behavior enables the safe migration of guest virtual machines between host physical machines, provided they all have physical CPUs that classify into the same group. libvirt does not typically enforce policy itself, rather it provides the mechanism on which the higher layers define their own desired policy. Understanding how to obtain CPU model information and define a suitable guest virtual machine CPU model is critical to ensure guest virtual machine migration is successful between host physical machines. Note that a hypervisor can only emulate features that it is aware of and features that were created after the hypervisor was released may not be emulated.