From 8bc7b950b7c0a3c80cdd120fe4df97c14848c344 Mon Sep 17 00:00:00 2001
From: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Date: Wed, 3 Jan 2018 17:44:27 +0100
Subject: [PATCH] libvirt: use 'host-passthrough' as default on AArch64

Nova is assuming 'host-model' for KVM/QEMU setup.

On AArch64 it results in "libvirtError: unsupported configuration: CPU
mode 'host-model' for aarch64 kvm domain on aarch64 host is not
supported by hypervisor" message.

AArch64 lacks 'host-model' support because neither libvirt nor QEMU
are able to tell what the host CPU model exactly is. And there is no
CPU description code for ARM(64) at this point.

So instead we fallback to 'host-passthrough' to get VM instances
running. This will completely break live migration, *unless* all the
Compute nodes (running libvirtd) have *identical* CPUs.

Small summary: https://marcin.juszkiewicz.com.pl/2018/01/04/today-i-was-fighting-with-nova-no-idea-who-won/

Closes-bug: #1741230

Co-authored-by: Kevin Zhao <Kevin.Zhao@arm.com>
Co-authored-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>

Change-Id: Iafb5f1790d68489db73b9f0549333108c6426a00
---
 nova/tests/unit/virt/libvirt/test_driver.py   | 32 +++++++++++++++++++
 nova/virt/libvirt/driver.py                   | 18 ++++++++++-
 ...-set-proper-cpu-mode-8455bad7d69dc6fd.yaml | 13 ++++++++
 3 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 releasenotes/notes/aarch64-set-proper-cpu-mode-8455bad7d69dc6fd.yaml

diff --git a/nova/tests/unit/virt/libvirt/test_driver.py b/nova/tests/unit/virt/libvirt/test_driver.py
index d2998bfbe590..298b182ec034 100755
--- a/nova/tests/unit/virt/libvirt/test_driver.py
+++ b/nova/tests/unit/virt/libvirt/test_driver.py
@@ -5826,6 +5826,38 @@ class LibvirtConnTestCase(test.NoDBTestCase,
         self.assertEqual(conf.cpu.cores, 1)
         self.assertEqual(conf.cpu.threads, 1)
 
+    def test_get_guest_cpu_config_host_passthrough_aarch64(self):
+        expected = {
+            fields.Architecture.X86_64: "host-model",
+            fields.Architecture.I686: "host-model",
+            fields.Architecture.PPC: "host-model",
+            fields.Architecture.PPC64: "host-model",
+            fields.Architecture.ARMV7: "host-model",
+            fields.Architecture.AARCH64: "host-passthrough",
+        }
+        for guestarch, expect_mode in expected.items():
+            caps = vconfig.LibvirtConfigCaps()
+            caps.host = vconfig.LibvirtConfigCapsHost()
+            caps.host.cpu = vconfig.LibvirtConfigCPU()
+            caps.host.cpu.arch = guestarch
+            with mock.patch.object(host.Host, "get_capabilities",
+                                   return_value=caps):
+                drvr = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), True)
+                if caps.host.cpu.arch == fields.Architecture.AARCH64:
+                    drvr._has_uefi_support = mock.Mock(return_value=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)
+                conf = drvr._get_guest_config(instance_ref,
+                                              _fake_network_info(self, 1),
+                                              image_meta, disk_info)
+                self.assertIsInstance(conf.cpu,
+                                      vconfig.LibvirtConfigGuestCPU)
+                self.assertEqual(conf.cpu.mode, expect_mode)
+
     def test_get_guest_cpu_config_host_model(self):
         drvr = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), True)
         instance_ref = objects.Instance(**self.test_instance)
diff --git a/nova/virt/libvirt/driver.py b/nova/virt/libvirt/driver.py
index ce24cd2be9bc..4209af1a62b0 100644
--- a/nova/virt/libvirt/driver.py
+++ b/nova/virt/libvirt/driver.py
@@ -3551,7 +3551,23 @@ class LibvirtDriver(driver.ComputeDriver):
         if (CONF.libvirt.virt_type == "kvm" or
             CONF.libvirt.virt_type == "qemu"):
             if mode is None:
-                mode = "host-model"
+                caps = self._host.get_capabilities()
+                # AArch64 lacks 'host-model' support because neither libvirt
+                # nor QEMU are able to tell what the host CPU model exactly is.
+                # And there is no CPU description code for ARM(64) at this
+                # point.
+
+                # Also worth noting: 'host-passthrough' mode will completely
+                # break live migration, *unless* all the Compute nodes (running
+                # libvirtd) have *identical* CPUs.
+                if caps.host.cpu.arch == fields.Architecture.AARCH64:
+                    mode = "host-passthrough"
+                    LOG.info('CPU mode "host-passthrough" was chosen. Live '
+                             'migration can break unless all compute nodes '
+                             'have identical cpus. AArch64 does not support '
+                             'other modes.')
+                else:
+                    mode = "host-model"
             if mode == "none":
                 return vconfig.LibvirtConfigGuestCPU()
         else:
diff --git a/releasenotes/notes/aarch64-set-proper-cpu-mode-8455bad7d69dc6fd.yaml b/releasenotes/notes/aarch64-set-proper-cpu-mode-8455bad7d69dc6fd.yaml
new file mode 100644
index 000000000000..6458fbe74c8e
--- /dev/null
+++ b/releasenotes/notes/aarch64-set-proper-cpu-mode-8455bad7d69dc6fd.yaml
@@ -0,0 +1,13 @@
+---
+upgrade:
+  - |
+    On AArch64 architecture ``cpu_mode`` for libvirt is set to ``host-passthrough``
+    by default.
+
+    AArch64 currently lacks ``host-model`` support because neither libvirt nor
+    QEMU are able to tell what the host CPU model exactly is and there is no
+    CPU description code for ARM(64) at this point.
+
+    .. warning:: ``host-passthrough`` mode will completely break live
+       migration, *unless* all the Compute nodes (running libvirtd) have
+       *identical* CPUs.