feat(instances): a VM's TPM 2.0 is switched on and off on its Options tab

A VM made without a TPM, a Windows 10 to be upgraded to Windows 11, had
no way to get one but the XML editor. The Options tab has a TPM 2.0
switch, as the RNG one: it adds an emulated TPM of the host's model to
the definition for the next start, or removes it. Its state stays on the
host until the VM is undefined, so a TPM added again has it back. A TPM
the switch did not make (passthrough, 1.2, encrypted, with its own state
path) is left to the XML editor, and a host without swtpm says so.

vrtManager/instance.py

@@ -2968,6 +2968,37 @@ class wvmInstance(wvmConnect):
             tree.find("devices").append(etree.fromstring(RNG_DEVICE))
         self._defineXML(etree.tostring(tree).decode())
 
+    def get_tpm(self):
+        """The persistent definition's TPM: "emulator" for a plain emulated
+        TPM 2.0, the panel's own, "custom" for another one, None without one."""
+        tpms = etree.fromstring(self._XMLDesc(PERSISTENT_XML)).xpath("devices/tpm")
+        if not tpms:
+            return None
+        # libvirt adds only an alias; encryption, a state path or another
+        # backend setting is kept by the XML editor, not dropped by the switch
+        own = ("backend[@type='emulator' and (not(@version) or @version='2.0') and not(*)"
+               " and not(@*[name()!='type' and name()!='version'])] and not(*[not(self::backend or self::alias)])")
+        if len(tpms) == 1 and tpms[0].xpath(own):
+            return "emulator"
+        return "custom"
+
+    def set_tpm(self, enabled):
+        """Adds an emulated TPM 2.0 to the persistent definition, or removes it;
+        another TPM is left to the XML editor. Its state stays on the host until
+        the VM is undefined, so a TPM added again has it back."""
+        if self.get_tpm() == "custom":
+            raise util.OperationError("The VM has its own TPM: change it in the XML editor")
+        tree = etree.fromstring(self._XMLDesc(PERSISTENT_XML))
+        for tpm in tree.xpath("devices/tpm"):
+            tpm.getparent().remove(tpm)
+        if enabled:
+            model = self.get_tpm_model(self.get_arch(), self.get_machine_type())
+            if not model:
+                raise util.OperationError("This host offers no TPM 2.0: install swtpm on it")
+            tree.find("devices").append(
+                etree.fromstring(f"<tpm model='{model}'><backend type='emulator' version='2.0'/></tpm>"))
+        self._defineXML(etree.tostring(tree).decode())
+
     def get_on_crash(self):
         """What the next start does when the guest crashes; libvirt's default is destroy."""
         return etree.fromstring(self._XMLDesc(PERSISTENT_XML)).findtext("on_crash") or "destroy"

vrtManager/tests/test_device_edits.py

@@ -176,6 +176,45 @@ class DeviceEditsTestCase(unittest.TestCase):
                         p.set_rng(False)
                     p._defineXML.assert_not_called()
 
+    def test_an_emulated_tpm_2_is_added_with_the_host_s_model_or_left_to_the_xml_editor(self):
+        p = proxy()
+        p.get_tpm_model = lambda arch, machine: "tpm-crb"
+        p.get_arch, p.get_machine_type = lambda: "x86_64", lambda: "q35"
+        self.assertIsNone(p.get_tpm())
+        p.set_tpm(True)
+        tpm = defined(p).find("devices/tpm")
+        self.assertEqual((tpm.get("model"), dict(tpm.find("backend").attrib)), ("tpm-crb", {"type": "emulator", "version": "2.0"}))
+        added = etree.tostring(defined(p)).decode()
+        p._XMLDesc = lambda flags: added
+        self.assertEqual(p.get_tpm(), "emulator")
+        p.set_tpm(False)
+        self.assertIsNone(defined(p).find("devices/tpm"))
+        # a host without swtpm adds none
+        p = proxy()
+        p.get_tpm_model = lambda arch, machine: None
+        p.get_arch, p.get_machine_type = lambda: "x86_64", lambda: "q35"
+        with self.assertRaises(util.OperationError):
+            p.set_tpm(True)
+        p._defineXML.assert_not_called()
+        # libvirt's alias leaves it the panel's
+        p = proxy()
+        p._XMLDesc = lambda flags: PERSISTENT.replace(
+            "<video>", "<tpm model='tpm-crb'><backend type='emulator' version='2.0'/><alias name='tpm0'/></tpm><video>")
+        self.assertEqual(p.get_tpm(), "emulator")
+        # a passthrough TPM, a TPM 1.2, an encrypted one or one with its own state path is the VM's own
+        for backend in ("<backend type='passthrough'><device path='/dev/tpm0'/></backend>",
+                        "<backend type='emulator' version='1.2'/>",
+                        "<backend type='emulator' version='2.0'><encryption secret='aaaaaaaa-0000-4000-8000-000000000001'/></backend>",
+                        "<backend type='emulator' version='2.0'><source type='dir' path='/srv/tpm'/></backend>",
+                        "<backend type='emulator' version='2.0' persistent_state='yes'/>"):
+            with self.subTest(backend):
+                p = proxy()
+                p._XMLDesc = lambda flags, b=backend: PERSISTENT.replace("<video>", f"<tpm model='tpm-tis'>{b}</tpm><video>")
+                self.assertEqual(p.get_tpm(), "custom")
+                with self.assertRaises(util.OperationError):
+                    p.set_tpm(False)
+                p._defineXML.assert_not_called()
+
     def test_the_crash_action_is_restart_or_stop(self):
         p = proxy()
         self.assertEqual(p.get_on_crash(), "destroy")  # libvirt's default