From ec48e1523dfdd0f1031e1a70c28c56962db66e8a Mon Sep 17 00:00:00 2001
From: Sean Mooney <work@seanmooney.info>
Date: Tue, 8 Dec 2020 16:42:52 +0000
Subject: [PATCH] workarounds: Add libvirt_disable_apic

This change adds a new workaround config option to allow disabling the
guest apic.

Co-authored-by: Lee Yarwood <lyarwood@redhat.com>
Related-Bug: #1939108
Change-Id: I2da867f2734b590a884b1fe1200c402cbf7e9e1c
---
 nova/conf/workarounds.py                      | 10 ++++++
 nova/tests/unit/virt/libvirt/test_driver.py   | 31 +++++++++++++++++++
 nova/virt/libvirt/driver.py                   |  3 +-
 ...libvirt-disable-apic-39599bdc2d110a1f.yaml | 13 ++++++++
 4 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 releasenotes/notes/libvirt-disable-apic-39599bdc2d110a1f.yaml

diff --git a/nova/conf/workarounds.py b/nova/conf/workarounds.py
index 53a9f3d417a0..4e51c8cc79f9 100644
--- a/nova/conf/workarounds.py
+++ b/nova/conf/workarounds.py
@@ -353,6 +353,16 @@ Related options:
 * :oslo.config:option:`DEFAULT.instances_path`
 * :oslo.config:option:`image_cache.subdirectory_name`
 * :oslo.config:option:`update_resources_interval`
+"""),
+    cfg.BoolOpt('libvirt_disable_apic',
+               default=False,
+               help="""
+With some kernels initializing the guest apic can result in a kernel hang that
+renders the guest unusable. This happens as a result of a kernel bug. In most
+cases the correct fix it to update the guest image kernel to one that is
+patched however in some cases this is not possible. This workaround allows the
+emulation of an apic to be disabled per host however it is not recommended to
+use outside of a CI or developer cloud.
 """),
 ]
 
diff --git a/nova/tests/unit/virt/libvirt/test_driver.py b/nova/tests/unit/virt/libvirt/test_driver.py
index 49b4773fc9d8..9eb75c495252 100644
--- a/nova/tests/unit/virt/libvirt/test_driver.py
+++ b/nova/tests/unit/virt/libvirt/test_driver.py
@@ -4917,6 +4917,37 @@ class LibvirtConnTestCase(test.NoDBTestCase,
         self.assertTrue(cfg.features[2].vapic)
         self.assertEqual(hvid_hidden, cfg.features[2].vendorid_spoof)
 
+    @mock.patch.object(host.Host, 'has_min_version',
+                       new=mock.Mock(return_value=True))
+    def test_get_guest_config_apic_workaround(self):
+        self.flags(virt_type='qemu', group='libvirt')
+        drvr = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), True)
+        instance_ref = objects.Instance(**self.test_instance)
+        image_meta = objects.ImageMeta.from_dict(self.test_image_meta)
+        disk_info = blockinfo.get_disk_info(
+            CONF.libvirt.virt_type, instance_ref, image_meta)
+
+        cfg = drvr._get_guest_config(
+            instance_ref, _fake_network_info(self), image_meta, disk_info)
+
+        self.assertEqual(3, len(cfg.features))
+        self.assertIsInstance(
+            cfg.features[0], vconfig.LibvirtConfigGuestFeatureACPI)
+        self.assertIsInstance(
+            cfg.features[1], vconfig.LibvirtConfigGuestFeatureAPIC)
+        self.assertIsInstance(
+            cfg.features[2], vconfig.LibvirtConfigGuestFeatureVMCoreInfo)
+
+        self.flags(libvirt_disable_apic=True, group='workarounds')
+        cfg = drvr._get_guest_config(
+            instance_ref, _fake_network_info(self), image_meta, disk_info)
+
+        self.assertEqual(2, len(cfg.features))
+        self.assertIsInstance(
+            cfg.features[0], vconfig.LibvirtConfigGuestFeatureACPI)
+        self.assertIsInstance(
+            cfg.features[1], vconfig.LibvirtConfigGuestFeatureVMCoreInfo)
+
     def test_get_guest_config_windows_hyperv_feature2(self):
         self._test_get_guest_config_windows_hyperv()
 
diff --git a/nova/virt/libvirt/driver.py b/nova/virt/libvirt/driver.py
index 8d544c9118e4..e7e2c5536400 100644
--- a/nova/virt/libvirt/driver.py
+++ b/nova/virt/libvirt/driver.py
@@ -5859,7 +5859,8 @@ class LibvirtDriver(driver.ComputeDriver):
 
         if CONF.libvirt.virt_type in ('qemu', 'kvm'):
             guest.features.append(vconfig.LibvirtConfigGuestFeatureACPI())
-            guest.features.append(vconfig.LibvirtConfigGuestFeatureAPIC())
+            if not CONF.workarounds.libvirt_disable_apic:
+                guest.features.append(vconfig.LibvirtConfigGuestFeatureAPIC())
 
         if CONF.libvirt.virt_type in ('qemu', 'kvm') and os_type == 'windows':
             hv = vconfig.LibvirtConfigGuestFeatureHyperV()
diff --git a/releasenotes/notes/libvirt-disable-apic-39599bdc2d110a1f.yaml b/releasenotes/notes/libvirt-disable-apic-39599bdc2d110a1f.yaml
new file mode 100644
index 000000000000..83ab5d7a034c
--- /dev/null
+++ b/releasenotes/notes/libvirt-disable-apic-39599bdc2d110a1f.yaml
@@ -0,0 +1,13 @@
+---
+issues:
+  - |
+    Linux guest images that have known kernel bugs related to virtualized apic
+    initialization previously would sporadically hang. For images where the
+    kernel cannot be upgraded, a ``[workarounds]`` config option has been
+    introduced:
+
+    ``[workarounds]libvirt_disable_apic``
+
+    This option is primarily intended for CI and development clouds as a bridge
+    for operators to mitigate the issue while they work with their upstream
+    image vendors.