Kvm cores option a lie?


I’ve been told recently that the “cores” option to KVM does not restrict the VM from running threads on other cpus and cores. I’m told that Intel technology does not support this and that it is impossible to actually have a VM run on only one core at a time. I’m further told that setting the cores option to anything other than “cores=1” will actually decrease the performance of the VM.

I’ve been doing KVM virtualization ever since RedHat began supporting it and I’ve never heard of this. I can’t find documentation to support these statements, but I can’t find any to refute it either. Can anyone point me to specific documentation on this?