libvirt: Add a workaround to skip compareCPU() on destination

Nova's use of libvirt's compareCPU() API served its purpose
over the years, but its design limitations break live migration in
subtle ways.  For example, the compareCPU() API compares against the
host physical CPUID.  Some of the features from this CPUID aren not
exposed by KVM, and then there are some features that KVM emulates that
are not in the host CPUID.  The latter can cause bogus live migration
failures.

With QEMU >=2.9 and libvirt >= 4.4.0, libvirt will do the right thing in
terms of CPU compatibility checks on the destination host during live
migration.  Nova satisfies these minimum version requirements by a good
margin.  So, provide a workaround to skip the CPU comparison check on
the destination host before migrating a guest, and let libvirt handle it
correctly.  This workaround will be removed once Nova replaces the older
libvirt APIs with their newer and improved counterparts[1][2].

                - - -

Note that Nova's libvirt driver calls compareCPU() in another method,
_check_cpu_compatibility(); I did not remove its usage yet.  As it needs
more careful combing of the code, and then:

  - where possible, remove the usage of compareCPU() altogether, and
    rely on libvirt doing the right thing under the hood; or

  - where Nova _must_ do the CPU comparison checks, switch to the better
    libvirt CPU APIs -- baselineHypervisorCPU() and
    compareHypervisorCPU() -- that are described here[1].  This is work
    in progress[2].

[1] https://opendev.org/openstack/nova-specs/commit/70811da221035044e27
[2] https://review.opendev.org/q/topic:bp%252Fcpu-selection-with-hypervisor-consideration

Change-Id: I444991584118a969e9ea04d352821b07ec0ba88d
Closes-Bug: #1913716
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Balazs Gibizer <bgibizer@redhat.com>
(cherry picked from commit 267a40663c)

nova/conf/workarounds.py

@@ -401,6 +401,14 @@ VCPU and PCPU resource usage counts should set this option to True.
 Related options:
 
 * :oslo.config:option:`quota.driver`
+"""),
+    cfg.BoolOpt('skip_cpu_compare_on_dest',
+               default=False,
+               help="""
+With the libvirt driver, during live migration, skip comparing guest CPU
+with the destination host. When using QEMU >= 2.9 and libvirt >=
+4.4.0, libvirt will do the correct thing with respect to checking CPU
+compatibility on the destination host during live migration.
 """),
 ]
 

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

@@ -10915,6 +10915,25 @@ class LibvirtConnTestCase(test.NoDBTestCase,
                           'serial_listen_addr': None},
                          result.obj_to_primitive()['nova_object.data'])
 
+    @mock.patch(
+        'nova.network.neutron.API.has_port_binding_extension',
+        new=mock.Mock(return_value=False))
+    @mock.patch.object(libvirt_driver.LibvirtDriver,
+                       '_create_shared_storage_test_file',
+                       return_value='fake')
+    @mock.patch.object(libvirt_driver.LibvirtDriver, '_compare_cpu')
+    def test_check_can_live_migrate_guest_cpu_none_model_skip_compare(
+            self, mock_cpu, mock_test_file):
+        self.flags(group='workarounds', skip_cpu_compare_on_dest=True)
+        instance_ref = objects.Instance(**self.test_instance)
+        instance_ref.vcpu_model = test_vcpu_model.fake_vcpumodel
+        instance_ref.vcpu_model.model = None
+        drvr = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), False)
+        compute_info = {'cpu_info': 'asdf', 'disk_available_least': 1}
+        drvr.check_can_live_migrate_destination(
+            self.context, instance_ref, compute_info, compute_info)
+        mock_cpu.assert_not_called()
+
     @mock.patch(
         'nova.network.neutron.API.has_port_binding_extension',
         new=mock.Mock(return_value=False))

nova/virt/libvirt/driver.py

@@ -9338,15 +9338,16 @@ class LibvirtDriver(driver.ComputeDriver):
         disk_available_mb = (
             (disk_available_gb * units.Ki) - CONF.reserved_host_disk_mb)
 
-        # Compare CPU
-        try:
-            if not instance.vcpu_model or not instance.vcpu_model.model:
-                source_cpu_info = src_compute_info['cpu_info']
-                self._compare_cpu(None, source_cpu_info, instance)
-            else:
-                self._compare_cpu(instance.vcpu_model, None, instance)
-        except exception.InvalidCPUInfo as e:
-            raise exception.MigrationPreCheckError(reason=e)
+        if not CONF.workarounds.skip_cpu_compare_on_dest:
+            # Compare CPU
+            try:
+                if not instance.vcpu_model or not instance.vcpu_model.model:
+                    source_cpu_info = src_compute_info['cpu_info']
+                    self._compare_cpu(None, source_cpu_info, instance)
+                else:
+                    self._compare_cpu(instance.vcpu_model, None, instance)
+            except exception.InvalidCPUInfo as e:
+                raise exception.MigrationPreCheckError(reason=e)
 
         # Create file on storage, to be checked on source host
         filename = self._create_shared_storage_test_file(instance)

releasenotes/notes/skip-compare-cpu-on-dest-6ae419ddd61fd0f8.yaml

@@ -0,0 +1,24 @@
+---
+issues:
+  - |
+    Nova's use of libvirt's compareCPU() API served its purpose over the
+    years, but its design limitations break live migration in subtle
+    ways. For example, the compareCPU() API compares against the host
+    physical CPUID. Some of the features from this CPUID aren not
+    exposed by KVM, and then there are some features that KVM emulates
+    that are not in the host CPUID. The latter can cause bogus live
+    migration failures.
+
+    With QEMU >=2.9 and libvirt >= 4.4.0, libvirt will do the right
+    thing in terms of CPU compatibility checks on the destination host
+    during live migration. Nova satisfies these minimum version
+    requirements by a good margin. So, this workaround provides a way to
+    skip the CPU comparison check on the destination host before
+    migrating a guest, and let libvirt handle it correctly.
+
+    This workaround will be deprecated and removed once Nova replaces
+    the older libvirt APIs with their newer counterparts. The work is
+    being tracked via this `blueprint
+    cpu-selection-with-hypervisor-consideration`_.
+
+    .. _blueprint cpu-selection-with-hypervisor-consideration: https://blueprints.launchpad.net/nova/+spec/cpu-selection-with-hypervisor-consideration