Comment 4 for bug 1754704

Note that there is no setting to have this behavior. The exclustion requires first a set of cores:

Comment in /usr/lib/python2.7/dist-packages/nova/virt/hardware.py:
    Each element in the list is either a single
    CPU number, a range of CPU numbers, or a
    caret followed by a CPU number to be excluded
    from a previous range.

So there is no way to specify vcpu pin set core "7-last", one has to always specify a range of all cores to be used. So if the core count varies between nodes, each set of nodes have to have separate juju application deployed with separate vcpu-pin-set.

Note also that the parameter description is misleading:
      Sets vcpu_pin_set option in nova.conf which defines which pcpus that
      instance vcpus can or cannot use. For example '^0,^2' to reserve two
      cpus for the host.

That would indicate that ^0-6 should work correctly, reserving cores 0-6 for host only. However if you take into account the comment in hardware.py, this should not work, because you first need to specify a range of all cores first...