Avoid setting cpu element on unsupported architectures (#1633)

The CPU element to manage the mode, model, features (including nested),
is only available on some architectures. To allow this plugin to
generate XML valid for other architectures such as RISC-V, the CPU
element needs to be optional and only enabled when the architecture
specified supports it.

Include checks in the validation section to help prevent the setting of
an unsupported architecture with any of the CPU features that require
the CPU element to be available.

Fixes: #1538

spec/unit/action/start_domain_spec.rb

@@ -161,6 +161,71 @@ describe VagrantPlugins::ProviderLibvirt::Action::StartDomain do
       end
     end
 
+    context 'cpu' do
+      let(:test_file) { 'existing.xml' }
+      let(:updated_domain_xml) {
+        new_xml = domain_xml.dup
+        new_xml.gsub!(
+          /<cpu .*\/>/,
+          <<-EOF
+          <cpu check='partial' mode='custom'>
+            <model fallback='allow'>Haswell</model>
+            <feature name='vmx' policy='optional'/>
+            <feature name='svm' policy='optional'/>
+          </cpu>
+          EOF
+        )
+        new_xml
+      }
+      let(:vagrantfile_providerconfig) {
+        <<-EOF
+        libvirt.cpu_mode = 'custom'
+        libvirt.cpu_model = 'Haswell'
+        libvirt.nested = true
+        EOF
+      }
+
+      it 'should set cpu related settings when changed' do
+        expect(ui).to_not receive(:warn)
+        expect(connection).to receive(:define_domain).and_return(libvirt_domain)
+        expect(libvirt_domain).to receive(:xml_desc).and_return(domain_xml, updated_domain_xml)
+        expect(libvirt_domain).to receive(:autostart=)
+        expect(domain).to receive(:start)
+
+        expect(subject.call(env)).to be_nil
+      end
+
+      let(:domain_xml_no_cpu) {
+        new_xml = domain_xml.dup
+        new_xml.gsub!(/<cpu .*\/>/, '')
+        new_xml
+      }
+      let(:updated_domain_xml_new_cpu) {
+        new_xml = domain_xml.dup
+        new_xml.gsub!(
+          /<cpu .*\/>/,
+          <<-EOF
+          <cpu mode='custom'>
+            <model fallback='allow'>Haswell</model>
+            <feature name='vmx' policy='optional'/>
+            <feature name='svm' policy='optional'/>
+          </cpu>
+          EOF
+        )
+        new_xml
+      }
+
+      it 'should add cpu settings if not already present' do
+        expect(ui).to_not receive(:warn)
+        expect(connection).to receive(:define_domain).and_return(libvirt_domain)
+        expect(libvirt_domain).to receive(:xml_desc).and_return(domain_xml_no_cpu, updated_domain_xml_new_cpu)
+        expect(libvirt_domain).to receive(:autostart=)
+        expect(domain).to receive(:start)
+
+        expect(subject.call(env)).to be_nil
+      end
+    end
+
     context 'graphics' do
       context 'autoport not disabled' do
         let(:test_file) { 'existing.xml' }

spec/unit/config_spec.rb

@@ -676,6 +676,33 @@ describe VagrantPlugins::ProviderLibvirt::Config do
         expect(subject.channels).to match([a_hash_including({:target_name => 'com.redhat.spice.0'})])
       end
     end
+
+    context '@machine_arch and @cpu_*' do
+      context 'should set @cpu_mode based on @machine_arch support' do
+        # it's possible when this is unset that the host arch should be read
+        it 'should default to host-model if machine_arch unset' do
+          subject.finalize!
+
+          expect(subject.cpu_mode).to eq('host-model')
+        end
+
+        it 'should default to host-model if supported' do
+          subject.machine_arch = 'aarch64'
+
+          subject.finalize!
+
+          expect(subject.cpu_mode).to eq('host-model')
+        end
+
+        it 'should default to nil if unsupported' do
+          subject.machine_arch = 'ppc'
+
+          subject.finalize!
+
+          expect(subject.cpu_mode).to be_nil
+        end
+      end
+    end
   end
 
   def assert_invalid
@@ -804,6 +831,32 @@ describe VagrantPlugins::ProviderLibvirt::Config do
       end
     end
 
+    context '@machine_arch and @cpu_*' do
+      it 'should be valid if cpu_* settings and no arch set' do
+        subject.cpu_mode = 'host-passthrough'
+        subject.nested = true
+
+        assert_valid
+      end
+
+      it 'should be valid if cpu_* settings and supported' do
+        subject.machine_arch = 'aarch64'
+        subject.cpu_mode = 'host-passthrough'
+        subject.nested = true
+
+        assert_valid
+      end
+
+      it 'should flag settings invalid if unsupported' do
+        subject.machine_arch = 'ppc'
+        subject.cpu_mode = 'host-passthrough'
+        subject.nested = true
+
+        errors = assert_invalid
+        expect(errors).to include(match(/Architecture ppc does not support .* cpu_mode, nested/))
+      end
+    end
+
     context 'with cpu_mode and cpu_model defined' do
       it 'should discard model if mode is passthrough' do
         subject.cpu_mode = 'host-passthrough'