vcpu_pin_set policy

This sounds quite a sensible approach - the current approach does depend on all compute hosts being the same spec which is not awesome.

Is it possible todo:

vcpu-pin-set: "^0-6"

As that has the same effect as 7-last without the need to calculate the number of CPU's

I have tried using the vcpu-pin-set: "^0-6":

dtadmin@zag0t1a-mgm231control-hr:~$ juju config nova-compute-ext-dpdk vcpu-pin-set="^0-6"
dtadmin@zag0t1a-mgm231control-hr:~$ juju config nova-compute-ext-dpdk vcpu-pin-set
^0-6

This does not work however:

2018-05-22 08:44:20.988 21097 ERROR nova.compute.manager Invalid: No CPUs available after parsing '^0-6'

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...