Description of problem:
In code here: https://opendev.org/openstack/nova/src/branch/stable/train/nova/virt/libvirt/vif.py#L207-L241 the number of tap queues is determined:

            driver = 'vhost'
            max_tap_queues = self._get_max_tap_queues()

The function _get_max_tap_queues() returns a number based on the major kernel version.

There is no real distinction between the type of ports (i.e. vhostuser), so also for fast datapath on kernel 3, 8 will be returned anyway. "When the domain XML is written for the guest, vhost_queues is used in the 'queues' argument in the driver. When this value is >8, it fails when attempting to create the tap interface." and fails to start the VM[1].

We have a customer looking into the possibility of having more than 8 queues on fast datapath on kernel 3 (RHOSP 13).


Version-Release number of selected component (if applicable):
RHOSP 13, 14, 15


Additional info:
May qualify as RFE or NOTABUG if this scenario (> 8 queues, fast datapath) has been tested by us already with different combinations.

[1] https://access.redhat.com/solutions/3227911