Why does nova limit the number of queues to 8 when booting an instance with a flavor extra spec of "hw:vif_multiqueue_enabled": "true" and more than 8 vCPUs in Red Hat OpenStack Platform?

Solution In Progress - Updated -

Issue

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Why does nova limit the number of queues to 8 when booting an instance with a flavor extra spec of "hw:vif_multiqueue_enabled": "true" and more than 8 vCPUs in Red Hat OpenStack Platform?

Flavor's extra specs:

[stack@undercloud-4 ~]$ nova flavor-show m1.small.12cpu | grep extra| extra_specs                | {"hw:cpu_policy": "dedicated", "hw:vif_multiqueue_enabled": "true", "hw:mem_page_size": "large", "hw:numa_nodes": "2"} |

Booting an instance with this flavor:

nova boot --nic net-id=$NETID --image rhel --flavor m1.small.12cpu --key-name id_rsa rhel-test2

This generates the following XML:

virsh dumpxml 16 | less
(...)
    <interface type='vhostuser'>
      <mac address='fa:16:3e:b7:76:4e'/>
      <source type='unix' path='/var/run/openvswitch/vhu1e79eee9-37' mode='client'/>
      <model type='virtio'/>
      <driver name='vhost' queues='8'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
(...)

And from nova-compute.log

[root@overcloud-compute-0 ~]# grep queue /var/log/nova/nova-compute.log | grep vhost | grep driver | tail -n1
      <driver name="vhost" queues="8"/>

Environment

Red Hat OpenStack Platform
Red Hat Enterprise Linux 7
Red Hat Enterprise Linux with 3.x kernel versions

This article is applicable in OSP 10 when nova/neutron uses tap interfaces to connect an instance to the virtual network.
This article is applicable in OSP 13 when nova/neutron uses tap interfaces to connect an instance to the virtual network
While this article covers nova/neutron networking with tap interfaces, setups with OVS-DPDK have the same 8 queue limit

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