Fix 1vcpu error with multiqueue and vif_type=tap

Fix for bug #1893263 introduced a regression where
1 vcpu instances would fail to build when paired with
multiqueue-enabled images, in the scenario vif_type=tap.

Solution is to not pass multiqueue parameter when
instances.get_flavor().vcpus = 1.

Closes-bug: #1939604
Change-Id: Iaccf2eeeb6e8bb80c658f51ce9ab4e8eb4093a55
(cherry picked from commit 7fc6fe6fae891eae42b36ccb9d69cd0f6d6db21d)
(cherry picked from commit aa5b8d12bcacc01e5f9be45cc1eef24ac9efd2fc)
(cherry picked from commit aaa56240b0311ad47ccccc3b7850ddc5b0a21702)

3 files changed, 43 insertions, 13 deletions

diff --git a/nova/tests/unit/virt/libvirt/test_vif.py b/nova/tests/unit/virt/libvirt/test_vif.py
index dcb8dba990..240f6143d5 100644
--- a/nova/tests/unit/virt/libvirt/test_vif.py
+++ b/nova/tests/unit/virt/libvirt/test_vif.py
@@ -380,6 +380,10 @@ class LibvirtVifTestCase(test.NoDBTestCase):
         uuid='f0000000-0000-0000-0000-000000000001',
         project_id=723)
 
+    flavor_1vcpu = objects.Flavor(vcpus=1, memory=512, root_gb=1)
+
+    flavor_2vcpu = objects.Flavor(vcpus=2, memory=512, root_gb=1)
+
     bandwidth = {
         'quota:vif_inbound_peak': '200',
         'quota:vif_outbound_peak': '20',
@@ -1091,30 +1095,50 @@ class LibvirtVifTestCase(test.NoDBTestCase):
     @mock.patch('nova.privsep.linux_net.device_exists', return_value=True)
     @mock.patch('nova.privsep.linux_net.set_device_mtu')
     @mock.patch('nova.privsep.linux_net.create_tap_dev')
-    def test_plug_tap_kvm_virtio(self, mock_create_tap_dev, mock_set_mtu,
-                      mock_device_exists):
+    def test_plug_tap_kvm_virtio(
+            self, mock_create_tap_dev, mock_set_mtu, mock_device_exists):
 
         d1 = vif.LibvirtGenericVIFDriver()
         ins = objects.Instance(
             id=1, uuid='f0000000-0000-0000-0000-000000000001',
+            flavor=self.flavor_2vcpu,
             project_id=723, system_metadata={}
         )
         d1.plug(ins, self.vif_tap)
-        mock_create_tap_dev.assert_called_once_with('tap-xxx-yyy-zzz', None,
-                                                    multiqueue=False)
+        mock_create_tap_dev.assert_called_once_with(
+            'tap-xxx-yyy-zzz', None, multiqueue=False)
 
         mock_create_tap_dev.reset_mock()
 
         d2 = vif.LibvirtGenericVIFDriver()
         mq_ins = objects.Instance(
             id=1, uuid='f0000000-0000-0000-0000-000000000001',
+            flavor=self.flavor_2vcpu,
             project_id=723, system_metadata={
                 'image_hw_vif_multiqueue_enabled': 'True'
             }
         )
         d2.plug(mq_ins, self.vif_tap)
-        mock_create_tap_dev.assert_called_once_with('tap-xxx-yyy-zzz', None,
-                                                    multiqueue=True)
+        mock_create_tap_dev.assert_called_once_with(
+            'tap-xxx-yyy-zzz', None, multiqueue=True)
+
+    @mock.patch('nova.privsep.linux_net.device_exists', return_value=True)
+    @mock.patch('nova.privsep.linux_net.set_device_mtu')
+    @mock.patch('nova.privsep.linux_net.create_tap_dev')
+    def test_plug_tap_mq_ignored_1vcpu(
+            self, mock_create_tap_dev, mock_set_mtu, mock_device_exists):
+
+        d1 = vif.LibvirtGenericVIFDriver()
+        mq_ins = objects.Instance(
+            id=1, uuid='f0000000-0000-0000-0000-000000000001',
+            image_ref=uuids.image_ref, flavor=self.flavor_1vcpu,
+            project_id=723, system_metadata={
+                'image_hw_vif_multiqueue_enabled': 'True',
+            }
+        )
+        d1.plug(mq_ins, self.vif_tap)
+        mock_create_tap_dev.assert_called_once_with(
+            'tap-xxx-yyy-zzz', None, multiqueue=False)
 
     @mock.patch('nova.privsep.linux_net.device_exists', return_value=True)
     @mock.patch('nova.privsep.linux_net.set_device_mtu')
@@ -1129,14 +1153,14 @@ class LibvirtVifTestCase(test.NoDBTestCase):
         d1 = vif.LibvirtGenericVIFDriver()
         ins = objects.Instance(
             id=1, uuid='f0000000-0000-0000-0000-000000000001',
+            flavor=self.flavor_2vcpu,
             project_id=723, system_metadata={
                 'image_hw_vif_multiqueue_enabled': 'True'
             }
         )
         d1.plug(ins, self.vif_tap)
-        mock_create_tap_dev.assert_called_once_with('tap-xxx-yyy-zzz',
-                                                    None,
-                                                    multiqueue=False)
+        mock_create_tap_dev.assert_called_once_with(
+            'tap-xxx-yyy-zzz', None, multiqueue=False)
 
     @mock.patch('nova.privsep.linux_net.device_exists', return_value=True)
     @mock.patch('nova.privsep.linux_net.set_device_mtu')
@@ -1147,15 +1171,15 @@ class LibvirtVifTestCase(test.NoDBTestCase):
         d1 = vif.LibvirtGenericVIFDriver()
         ins = objects.Instance(
             id=1, uuid='f0000000-0000-0000-0000-000000000001',
+            flavor=self.flavor_2vcpu,
             project_id=723, system_metadata={
                 'image_hw_vif_multiqueue_enabled': 'True',
                 'image_hw_vif_model': 'e1000',
             }
         )
         d1.plug(ins, self.vif_tap)
-        mock_create_tap_dev.assert_called_once_with('tap-xxx-yyy-zzz',
-                                                    None,
-                                                    multiqueue=False)
+        mock_create_tap_dev.assert_called_once_with(
+            'tap-xxx-yyy-zzz', None, multiqueue=False)
 
     def test_unplug_tap(self):
         d = vif.LibvirtGenericVIFDriver()
diff --git a/nova/virt/libvirt/vif.py b/nova/virt/libvirt/vif.py
index cf8aff35a0..c40b23f0f8 100644
--- a/nova/virt/libvirt/vif.py
+++ b/nova/virt/libvirt/vif.py
@@ -722,7 +722,8 @@ class LibvirtGenericVIFDriver(object):
         vif_model = self.get_vif_model(image_meta=image_meta)
         # TODO(ganso): explore whether multiqueue works for other vif models
         # that go through this code path.
-        multiqueue = (self._requests_multiqueue(image_meta) and
+        multiqueue = (instance.get_flavor().vcpus > 1 and
+                      self._requests_multiqueue(image_meta) and
                       vif_model == network_model.VIF_MODEL_VIRTIO)
         nova.privsep.linux_net.create_tap_dev(dev, mac, multiqueue=multiqueue)
         network = vif.get('network')
diff --git a/releasenotes/notes/bug-1939604-547c729b7741831b.yaml b/releasenotes/notes/bug-1939604-547c729b7741831b.yaml
new file mode 100644
index 0000000000..e14327c285
--- /dev/null
+++ b/releasenotes/notes/bug-1939604-547c729b7741831b.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Addressed an issue that prevented instances with 1 vcpu using multiqueue
+    feature from being created successfully when their vif_type is TAP.