commit eeeca4ceff576beaa8558360c8a6a165d716f996
Author: Andrew Bonney <andrew.bonney@bbc.co.uk>
Date:   Tue Oct 6 14:42:38 2020 +0100

    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

diff --git a/nova/tests/unit/virt/libvirt/test_config.py b/nova/tests/unit/virt/libvirt/test_config.py
index f0c2fa3..9a9342e 100644
--- a/nova/tests/unit/virt/libvirt/test_config.py
+++ b/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"
diff --git a/nova/virt/libvirt/config.py b/nova/virt/libvirt/config.py
index 7cfdb42..ea52564 100644
--- a/nova/virt/libvirt/config.py
+++ b/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