Move libvirt calculation of machine type to utils.py

The libvirt driver contains some code to calculate the
default machine type given an architecture, by looking it up
in CONF.libvirt.hw_machine_type.

This code will need to be reused when introducing calls to
libvirt's getDomainCapabilities() API, which requires the
machine type as one of the parameters.  However those calls
will need to be made from nova.virt.libvirt.host.Host which
has no access to the driver, so move the machine type
calculation code into nova.virt.libvirt.utils so that it can
be reused by both classes.

Also add some unit tests, and warn when an invalid config
value is used.

blueprint: amd-sev-libvirt-support
Change-Id: I055918ff16766c5b106d794a111ad8af8ff9ab23

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

@@ -179,6 +179,10 @@ def cpu_features_to_traits(features):
     return libvirt_utils.cpu_features_to_traits(features)
 
 
+def get_default_machine_type(arch):
+    return libvirt_utils.get_default_machine_type(arch)
+
+
 def mdev_name2uuid(mdev_name):
     return libvirt_utils.mdev_name2uuid(mdev_name)
 

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

@@ -32,6 +32,7 @@ from nova import objects
 from nova.objects import fields as obj_fields
 import nova.privsep.fs
 from nova import test
+from nova.tests import fixtures as nova_fixtures
 from nova.tests.unit import fake_instance
 from nova.tests.unit.virt.libvirt import fakelibvirt
 from nova.virt.disk import api as disk
@@ -969,3 +970,29 @@ sunrpc /var/lib/nfs/rpc_pipefs rpc_pipefs rw,relatime 0 0
         disk_path, format = libvirt_utils.find_disk(guest)
         self.assertEqual('/test/disk', disk_path)
         self.assertEqual('ploop', format)
+
+    def test_machine_type_mappings(self):
+        self.useFixture(nova_fixtures.ConfPatcher(
+            group="libvirt", hw_machine_type=['x86_64=q35', 'i686=legacy']))
+        self.assertDictEqual({'x86_64': 'q35', 'i686': 'legacy'},
+                             libvirt_utils.machine_type_mappings())
+
+    def test_invalid_machine_type_mappings(self):
+        self.useFixture(nova_fixtures.ConfPatcher(
+            group="libvirt", hw_machine_type=['x86_64=q35', 'foo']))
+        self.assertDictEqual({'x86_64': 'q35'},
+                             libvirt_utils.machine_type_mappings())
+
+    def test_get_default_machine_type(self):
+        self.useFixture(nova_fixtures.ConfPatcher(
+            group="libvirt", hw_machine_type=['x86_64=q35', 'i686=legacy']))
+        self.assertEqual('q35',
+                         libvirt_utils.get_default_machine_type('x86_64'))
+
+    def test_get_default_machine_type_empty(self):
+        self.assertIsNone(libvirt_utils.get_default_machine_type('sparc'))
+
+    def test_get_default_machine_type_missing(self):
+        self.useFixture(nova_fixtures.ConfPatcher(
+            group="libvirt", hw_machine_type=['x86_64=q35', 'i686=legacy']))
+        self.assertIsNone(libvirt_utils.get_default_machine_type('sparc'))

nova/virt/libvirt/driver.py

@@ -4227,13 +4227,6 @@ class LibvirtDriver(driver.ComputeDriver):
 
         return meta
 
-    def _machine_type_mappings(self):
-        mappings = {}
-        for mapping in CONF.libvirt.hw_machine_type:
-            host_arch, _, machine_type = mapping.partition('=')
-            mappings[host_arch] = machine_type
-        return mappings
-
     def _get_machine_type(self, image_meta, caps):
         # The guest machine type can be set as an image metadata
         # property, or otherwise based on architecture-specific
@@ -4257,9 +4250,10 @@ class LibvirtDriver(driver.ComputeDriver):
                 mach_type = 's390-ccw-virtio'
 
             # If set in the config, use that as the default.
-            if CONF.libvirt.hw_machine_type:
-                mappings = self._machine_type_mappings()
-                mach_type = mappings.get(caps.host.cpu.arch)
+            mach_type = (
+                libvirt_utils.get_default_machine_type(caps.host.cpu.arch)
+                or mach_type
+            )
 
         return mach_type
 

nova/virt/libvirt/utils.py

@@ -541,6 +541,21 @@ def get_cpu_model_from_arch(arch):
     return mode
 
 
+def machine_type_mappings():
+    mappings = {}
+    for mapping in CONF.libvirt.hw_machine_type or {}:
+        host_arch, _, machine_type = mapping.partition('=')
+        if machine_type == '':
+            LOG.warning("Invalid hw_machine_type config value %s", mapping)
+        else:
+            mappings[host_arch] = machine_type
+    return mappings
+
+
+def get_default_machine_type(arch):
+    return machine_type_mappings().get(arch)
+
+
 def mdev_name2uuid(mdev_name):
     """Convert an mdev name (of the form mdev_<uuid_with_underscores>) to a
     uuid (of the form 8-4-4-4-12).