Although CPU hotplug functionality is available in QEMU, the aarch64 Linux kernel is not yet able to handle the addition of new virtual CPUs to a running virtual machine. When QEMU is used to add a virtual CPU to a running virtual machine in KVM, and error is returned:

kvm_init_vcpu failed: Device or resource busy

CPU hotplug functionality is currently unavailable for UEK R5 on 64-bit Arm platforms. (Bug ID 28140386)