Handle disabled CPU features to fix live migration failures

When performing a live migration between hypervisors running
libvirt, where one or more CPU features are disabled, nova does
not take account of these. This results in migration failures
as none of the available hypervisor targets appear compatible.

This patch ensures that the libvirt 'disable' poicy is taken
account of, at least in a basic sense, by explicitly ignoring
items flagged in this way when enumerating CPU features.

Closes-Bug: #1898715
Change-Id: Iaf14ca97cfac99dd280d1114123f2d4bb6292b63

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

@@ -339,6 +339,26 @@ class LibvirtConfigCPUFeatureTest(LibvirtConfigBaseTest):
             <feature name="mtrr"/>
         """)
 
+    def test_config_parse_require(self):
+        xml = """
+            <feature name="mtrr" policy="require"/>
+        """
+        xmldoc = etree.fromstring(xml)
+        obj = config.LibvirtConfigCPUFeature()
+        obj.parse_dom(xmldoc)
+
+        self.assertEqual(obj.policy, "require")
+
+    def test_config_parse_disable(self):
+        xml = """
+            <feature name="mtrr" policy="disable"/>
+        """
+        xmldoc = etree.fromstring(xml)
+        obj = config.LibvirtConfigCPUFeature()
+        obj.parse_dom(xmldoc)
+
+        self.assertEqual(obj.policy, "disable")
+
 
 class LibvirtConfigGuestCPUFeatureTest(LibvirtConfigBaseTest):
 
@@ -437,6 +457,27 @@ class LibvirtConfigCPUTest(LibvirtConfigBaseTest):
             </cpu>
         """)
 
+    def test_config_disabled_features(self):
+        obj = config.LibvirtConfigCPU()
+        obj.model = "Penryn"
+        obj.vendor = "Intel"
+        obj.arch = obj_fields.Architecture.X86_64
+
+        disabled_feature = config.LibvirtConfigCPUFeature("mtrr")
+        disabled_feature.policy = "disable"
+        obj.add_feature(disabled_feature)
+        obj.add_feature(config.LibvirtConfigCPUFeature("apic"))
+
+        xml = obj.to_xml()
+        self.assertXmlEqual(xml, """
+            <cpu>
+              <arch>x86_64</arch>
+              <model>Penryn</model>
+              <vendor>Intel</vendor>
+              <feature name="apic"/>
+            </cpu>
+        """)
+
     def test_only_uniq_cpu_featues(self):
         obj = config.LibvirtConfigCPU()
         obj.model = "Penryn"

nova/virt/libvirt/config.py

@@ -674,11 +674,13 @@ class LibvirtConfigCPUFeature(LibvirtConfigObject):
                                                       **kwargs)
 
         self.name = name
+        self.policy = "require"
 
     def parse_dom(self, xmldoc):
         super(LibvirtConfigCPUFeature, self).parse_dom(xmldoc)
 
         self.name = xmldoc.get("name")
+        self.policy = xmldoc.get("policy", "require")
 
     def format_dom(self):
         ft = super(LibvirtConfigCPUFeature, self).format_dom()
@@ -730,7 +732,8 @@ class LibvirtConfigCPU(LibvirtConfigObject):
             elif c.tag == "feature":
                 f = LibvirtConfigCPUFeature()
                 f.parse_dom(c)
-                self.add_feature(f)
+                if f.policy != "disable":
+                    self.add_feature(f)
 
     def format_dom(self):
         cpu = super(LibvirtConfigCPU, self).format_dom()
@@ -753,7 +756,8 @@ class LibvirtConfigCPU(LibvirtConfigObject):
 
         # sorting the features to allow more predictable tests
         for f in sorted(self.features, key=lambda x: x.name):
-            cpu.append(f.format_dom())
+            if f.policy != "disable":
+                cpu.append(f.format_dom())
 
         return cpu