libvirt: avoid cpu check at s390x arch

nova compute will call check_can_live_migrate_destination when doing
live migration and it will compare cpu model, however, following info
indicated that cpu compare is not supported at s390x arch.

URI qemu:///system does not support full set of host capabilities: this
function is not supported by the connection driver:
cannot compute baseline CPU of s390x architecture

https://www.libvirt.org/news.html has the info
v5.9.0 has Improvements part indicated the compare was added at 5.9

so the workaround is to avoid the check and let the migration proceed.

Change-Id: I253f4f305ecf8b5331212be87caef41f2ebb747e
Closes-Bug: 1854126
(cherry picked from commit 011cce6adb)
(cherry picked from commit 7d7a3ba70a)

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

@@ -9348,6 +9348,26 @@ class LibvirtConnTestCase(test.NoDBTestCase,
         ret = conn._compare_cpu(None, None, instance)
         self.assertIsNone(ret)
 
+    def test_compare_cpu_virt_platform_s390x(self):
+        _fake_s390xcpu_info = {
+            "arch": "s390x",
+            "model": "test_model",
+            "vendor": "test_vendor",
+            "topology": {
+                "sockets": 1,
+                "cores": 8,
+                "threads": 16
+            },
+            "features": ["feature1", "feature2"]
+        }
+
+        instance = objects.Instance(**self.test_instance)
+        conn = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), False)
+        ret = conn._compare_cpu(None,
+                                jsonutils.dumps(_fake_s390xcpu_info),
+                                instance)
+        self.assertIsNone(ret)
+
     @mock.patch.object(host.Host, 'compare_cpu')
     @mock.patch.object(nova.virt.libvirt, 'config')
     def test_compare_cpu_invalid_cpuinfo_raises(self, mock_vconfig,

nova/virt/libvirt/driver.py

@@ -97,6 +97,7 @@ import nova.privsep.path
 import nova.privsep.utils
 from nova import utils
 from nova import version
+from nova.virt import arch
 from nova.virt import block_device as driver_block_device
 from nova.virt import configdrive
 from nova.virt.disk import api as disk_api
@@ -7434,6 +7435,17 @@ class LibvirtDriver(driver.ComputeDriver):
         else:
             cpu = self._vcpu_model_to_cpu_config(guest_cpu)
 
+        # s390x doesn't support cpu model in host info, so compare
+        # cpu info will raise an error anyway, thus have to avoid check
+        # see bug 1854126 for more info
+        min_libvirt_version = (5, 9, 0)
+        if (cpu.arch in (arch.S390X, arch.S390) and
+                not self._host.has_min_version(min_libvirt_version)):
+            LOG.debug("on s390x platform, the min libvirt version "
+                      "support cpu model compare is %s",
+                      min_libvirt_version)
+            return
+
         u = ("http://libvirt.org/html/libvirt-libvirt-host.html#"
              "virCPUCompareResult")
         m = _("CPU doesn't have compatibility.\n\n%(ret)s\n\nRefer to %(u)s")