From eeeca4ceff576beaa8558360c8a6a165d716f996 Mon Sep 17 00:00:00 2001
From: Andrew Bonney <andrew.bonney@bbc.co.uk>
Date: Tue, 6 Oct 2020 14:42:38 +0100
Subject: [PATCH] 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 | 41 +++++++++++++++++++++
 nova/virt/libvirt/config.py                 |  8 +++-
 2 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/nova/tests/unit/virt/libvirt/test_config.py b/nova/tests/unit/virt/libvirt/test_config.py
index f0c2fa3b51..9a9342efae 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 7cfdb4218b..ea525648b3 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