It will enshure that cpu_index for a given cpu stays the same regardless of the order cpus has been created/deleted. No compat code is needed as for initial cpus index in possible_cpus[] matches cpu_index that's been auto-allocated in cpu_exec_init(). Tha same applies for hotplug with cpu-add command if cpus are added sequentially in increasing order as 'id' matches cpu_index. If cpu-add had been used for creating out-of-order cpus, that created unmigratable instance since it were not possible to start target with the same cpu_index using old way of migrating instance with hotplugged cpus: * source QEMU with CLI (-smp 1,maxcpus=3 and cpu-add id=2) following set of cpu_index is allocated [0, 1] with apics set [0, 2] respectivelly * target QEMU is started with CLI -smp 2,maxcpus=3 resulting in set of cpu_index [0, 1] but with set of apics [0, 1] wich doesn't match source. So we don't need compat code in this case as it's never worked and newelly added device_add support would use stable cpu_index set by machine to begin with, so it won't have above limitation and source QEMU could be migrated to destination regardless of the order cpus were created. Signed-off-by: Igor Mammedov <imammedo@redhat.com> Reviewed-by: David Gibson <david@gibson.dropbear.id.au> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>