add [libvirt]/max_queues config option

This change adds a max_queues config option to allow
operators to set the maximium number of virtio queue
pairs that can be allocated to a virtio network
interface.

Change-Id: I9abe783a9a9443c799e7c74a57cc30835f679a01
Closes-Bug: #1847367
(cherry picked from commit 0e6aac3c2d)

nova/conf/libvirt.py

@@ -1319,6 +1319,14 @@ Configure virtio tx queue size.
 This option is only usable for virtio-net device with vhost-user
 backend. Available only with QEMU/KVM. Requires libvirt v3.7 QEMU
 v2.10."""),
+     cfg.IntOpt('max_queues', default=None, min=1, help="""
+The maximum number of virtio queue pairs that can be enabled
+when creating a multiqueue guest. The number of virtio queues
+allocated will be the lesser of the CPUs requested by the guest
+and the max value defined. By default, this value is set to none
+meaning the legacy limits based on the reported kernel
+major version will be used.
+"""),
 
 ]
 

nova/tests/unit/virt/libvirt/test_vif.py

@@ -657,6 +657,21 @@ class LibvirtVifTestCase(test.NoDBTestCase):
     def test_virtio_multiqueue_in_kernel_4(self, mock_uname):
         self._test_virtio_multiqueue(10, '10')
 
+    @mock.patch('os.uname', return_value=('Linux', '', '2.6.32-21-generic'))
+    def test_virtio_multiqueue_in_kernel_2_max_queues(self, mock_uname):
+        self.flags(max_queues=2, group='libvirt')
+        self._test_virtio_multiqueue(10, '2')
+
+    @mock.patch('os.uname', return_value=('Linux', '', '3.19.0-47-generic'))
+    def test_virtio_multiqueue_in_kernel_3_max_queues(self, mock_uname):
+        self.flags(max_queues=2, group='libvirt')
+        self._test_virtio_multiqueue(10, '2')
+
+    @mock.patch('os.uname', return_value=('Linux', '', '4.2.0-35-generic'))
+    def test_virtio_multiqueue_in_kernel_4_max_queues(self, mock_uname):
+        self.flags(max_queues=2, group='libvirt')
+        self._test_virtio_multiqueue(10, '2')
+
     def test_vhostuser_os_vif_multiqueue(self):
         d = vif.LibvirtGenericVIFDriver()
         hostimpl = host.Host("qemu:///system")

nova/virt/libvirt/vif.py

@@ -225,6 +225,13 @@ class LibvirtGenericVIFDriver(object):
         return (driver, vhost_queues)
 
     def _get_max_tap_queues(self):
+        # Note(sean-k-mooney): some linux distros have backported
+        # changes for newer kernels which make the kernel version
+        # number unreliable to determine the max queues supported
+        # To address this without making the code distro dependent
+        # we introduce a new config option and prefer it if set.
+        if CONF.libvirt.max_queues:
+            return CONF.libvirt.max_queues
         # NOTE(kengo.sakai): In kernels prior to 3.0,
         # multiple queues on a tap interface is not supported.
         # In kernels 3.x, the number of queues on a tap interface

releasenotes/notes/virtio-max-queues-27f73e988c7e66ba.yaml

@@ -0,0 +1,15 @@
+---
+other:
+  - |
+    The nova libvirt virt driver supports creating instances with multi-queue
+    virtio network interfaces. In previous releases nova has based the maximum
+    number of virtio queue pairs that can be allocated on the reported kernel
+    major version. It has been reported in `bug #1847367`_ that some distros have
+    backported changes from later major versions that make major version
+    number no longer suitable to determine the maximum virtio queue pair count.
+    A new config option has been added to the libvirt section of the nova.conf.
+    When defined nova will now use the ``[libvirt]/max_queues`` option to
+    define the max queues that can be configured, if undefined it will
+    fallback to the previous kernel version approach.
+
+    .. _`bug #1847367`: https://bugs.launchpad.net/nova/+bug/1847367