Check if flavor.vcpus is more than MAX_TAP_QUEUES

When attempting to instantiate an instance based on an image with
the metadata hw:vif_multiqueue_enabled=true, the code uses
flavor.vcpus as the number of queues on a tap interface.

In kernels prior to 3.0, multiple queues on a tap interface
is not supported[1]. In kernels 3.x, the number of queues
on a tap interface is limited to 8 as MAX_TAP_QUEUES in tun
driver[2]. From 4.0, the number is 256[3]. If flavor.vcpus
is more than MAX_TAP_QUEUES, creating the tap interface
fails.

This commit adds logic to check if flavor.vcpus is more
than MAX_TAP_QUEUES and use MAX_TAP_QUEUES as the number
of queues if so.

[1]https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/drivers/net/tun.c?id=refs/tags/v2.6.32.71#n101
[2]https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/drivers/net/tun.c?id=refs/tags/v3.18.35#n118
[3]https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/drivers/net/tun.c?id=refs/tags/v4.1.26#n128

Change-Id: I2aa24e3cf550ff69909a2b4bc8be90641dbe3d69
Closes-Bug: #1570631

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

@@ -487,14 +487,14 @@ class LibvirtVifTestCase(test.NoDBTestCase):
         conf.add_device(nic)
         return conf.to_xml()
 
-    def test_virtio_multiqueue(self):
+    def _test_virtio_multiqueue(self, vcpus, want_queues):
         self.flags(use_virtio_for_bridges=True,
                    virt_type='kvm',
                    group='libvirt')
 
         flavor = objects.Flavor(name='m1.small',
                     memory_mb=128,
-                    vcpus=4,
+                    vcpus=vcpus,
                     root_gb=0,
                     ephemeral_gb=0,
                     swap=0,
@@ -515,7 +515,22 @@ class LibvirtVifTestCase(test.NoDBTestCase):
         driver = node.find("driver").get("name")
         self.assertEqual(driver, 'vhost')
         queues = node.find("driver").get("queues")
-        self.assertEqual(queues, '4')
+        self.assertEqual(queues, want_queues)
+
+    def test_virtio_multiqueue(self):
+        self._test_virtio_multiqueue(4, '4')
+
+    @mock.patch('os.uname', return_value=('Linux', '', '2.6.32-21-generic'))
+    def test_virtio_multiqueue_in_kernel_2(self, mock_uname):
+        self._test_virtio_multiqueue(10, '1')
+
+    @mock.patch('os.uname', return_value=('Linux', '', '3.19.0-47-generic'))
+    def test_virtio_multiqueue_in_kernel_3(self, mock_uname):
+        self._test_virtio_multiqueue(10, '8')
+
+    @mock.patch('os.uname', return_value=('Linux', '', '4.2.0-35-generic'))
+    def test_virtio_multiqueue_in_kernel_4(self, mock_uname):
+        self._test_virtio_multiqueue(10, '10')
 
     def test_multiple_nics(self):
         conf = self._get_conf()

nova/virt/libvirt/vif.py

@@ -148,10 +148,31 @@ class LibvirtGenericVIFDriver(object):
         img_props = image_meta.properties
         if img_props.get('hw_vif_multiqueue_enabled'):
             driver = 'vhost'
-            vhost_queues = flavor.vcpus
+            max_tap_queues = self._get_max_tap_queues()
+            if max_tap_queues:
+                vhost_queues = (max_tap_queues if flavor.vcpus > max_tap_queues
+                    else flavor.vcpus)
+            else:
+                vhost_queues = flavor.vcpus
 
         return (driver, vhost_queues)
 
+    def _get_max_tap_queues(self):
+        # 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
+        # is limited to 8. From 4.0, the number is 256.
+        # See: https://bugs.launchpad.net/nova/+bug/1570631
+        kernel_version = int(os.uname()[2].split(".")[0])
+        if kernel_version <= 2:
+            return 1
+        elif kernel_version == 3:
+            return 8
+        elif kernel_version == 4:
+            return 256
+        else:
+            return None
+
     def get_bridge_name(self, vif):
         return vif['network']['bridge']
 

releasenotes/notes/multiqueue-on-tap-interface-2c9e1504fa4590f4.yaml

@@ -0,0 +1,12 @@
+---
+fixes:
+  - |
+    When instantiating an instance based on an image with the metadata
+    hw_vif_multiqueue_enabled=true, if flavor.vcpus is less than the limit
+    of the number of queues on a tap interface in the kernel, nova uses
+    flavor.vcpus as the number of queues. if not, nova uses the limit.
+    The limits are as follows:
+
+      * kernels prior to 3.0: 1
+      * kernels 3.x: 8
+      * kernels 4.x: 256