Add possibility to override libvirt config

This is the equivalent of "custom config" from R3.x.

fixes QubesOS/qubes-issues#1798

doc/index.rst

@@ -26,6 +26,7 @@ manpages and API documentation. For primary user documentation, see
 .. toctree::
    :maxdepth: 1
 
+   libvirt
    autoxml
    manpages/index
 

doc/libvirt.rst

@@ -0,0 +1,114 @@
+Custom libvirt config
+=====================
+
+Starting from Qubes OS R4.0, libvirt domain config is generated using jinja
+templates. Those templates can be overridden by the user in a couple of ways.
+A basic knowledge of jinja template language and libvirt xml spec is needed.
+
+.. seealso::
+
+   https://libvirt.org/formatdomain.html
+      Format of the domain XML in libvirt.
+
+   http://jinja.pocoo.org/docs/dev/templates/
+      Template format documentation.
+
+File paths
+----------
+
+In order of increasing precedence: the main template, from which the config is
+generated is :file:`/usr/share/templates/libvirt/xen.xml`).
+The distributor may put a file at
+:file:`/usr/share/qubes/template/xen-dist.xml`) to override this file. 
+User may put a file at either
+:file:`/etc/qubes/templates/libvirt/xen-user.xml` or
+:file:`/etc/qubes/templates/libvirt/by-name/<name>.xml`, where ``<name>`` is
+full name of the domain. Wildcards are not supported but symlinks are.
+
+Jinja has a concept of template names, which basically is the path below some
+load point, which in Qubes' case is :file:`/etc/qubes/templates` and
+:file:`/usr/share/qubes/templates`. Thus names of those templates are
+respectively ``'libvirt/xen.xml'``, ``'libvirt/xen-dist.xml'``,
+``'libvirt/xen-user.xml'`` and ``'libvirt/by-name/<name>.xml'``.
+This will be important later.
+
+.. note::
+
+   Those who know jinja python API will know that the abovementioned locations
+   aren't the only possibilities. Yes, it's a lie, but a justified one.
+
+What to put in the template
+---------------------------
+
+In principle the user may put anything in the template and there is no attempt
+to constrain the user from doing stupid things. One obvious thing is to copy the
+original config file and make changes.
+
+.. code-block:: jinja
+
+   <domain type="xen">
+       <name>{{ vm.name }}</name>
+       ...
+
+The better way is to inherit from the original template and override any number
+of blocks. This is the point when we need the name of the original template.
+
+.. code-block:: jinja
+
+   {% extends 'libvirt/xen.xml' %}
+   {% block devices %}
+       {{ super() }}
+       <serial type='pty'>
+           <target port='0'/>
+       </serial>
+   {% endblock %}
+
+``{% extends %}`` specifies which template we inherit from. Then you may put any
+block by putting new content inside ``{% block %}{% endblock %}``.
+``{{ super() }}`` is substituted with original content of the block as specified
+in the parent template. Untouched blocks remain as they were.
+
+The example above adds serial device.
+
+Template API
+------------
+
+.. warning::
+
+   This API is provisional and subject to change at the minor releases until
+   further notice. No backwards compatibility is promised.
+
+Globals
+```````
+vm
+   the domain object (instance of subclass of
+   :py:class:`qubes.vm.qubesvm.QubesVM`)
+
+Filters
+```````
+
+No custom filters at the moment.
+
+Blocks in the default template
+``````````````````````````````
+basic
+   Contains ``<name>``, ``<uuid>``, ``<memory>``, ``<currentMemory>`` and
+   ``<vcpu>`` nodes.
+
+os
+   Contents of ``<os>`` node.
+
+features
+   Contents of ``<features>`` node.
+
+clock
+   Contains the ``<clock>`` node.
+
+on
+   Contains ``<on_*>`` nodes.
+
+devices
+   Contents of ``<devices>`` node.
+
+
+.. vim: ts=3 sts=3 sw=3 et

qubes/app.py

@@ -625,7 +625,10 @@ class Qubes(qubes.PropertyHolder):
 
         #: jinja2 environment for libvirt XML templates
         self.env = jinja2.Environment(
-            loader=jinja2.FileSystemLoader('/usr/share/qubes/templates'),
+            loader=jinja2.FileSystemLoader([
+                '/etc/qubes/templates',
+                '/usr/share/qubes/templates',
+            ]),
             undefined=jinja2.StrictUndefined)
 
         if load:

qubes/vm/__init__.py

@@ -269,8 +269,12 @@ class BaseVM(qubes.PropertyHolder):
         :param bool prepare_dvm: If we are in the process of preparing \
             DisposableVM
         '''
-        domain_config = self.app.env.get_template('libvirt/xen.xml').render(
-            vm=self, prepare_dvm=prepare_dvm)
+        domain_config = self.app.env.select_template([
+                'libvirt/xen/by-name/{}.xml'.format(self.name),
+                'libvirt/xen-user.xml',
+                'libvirt/xen-dist.xml',
+                'libvirt/xen.xml',
+            ]).render(vm=self, prepare_dvm=prepare_dvm)
         return domain_config
 
 

templates/libvirt/xen.xml

@@ -1,10 +1,13 @@
 <domain type="xen">
+    {% block basic %}
         <name>{% if prepare_dvm %}%NAME%{% else %}{{ vm.name }}{% endif %}</name>
         <uuid>{{ vm.uuid }}</uuid>
         <memory unit="MiB">{{ vm.maxmem }}</memory>
         <currentMemory unit="MiB">{{ vm.memory }}</currentMemory>
         <vcpu placement="static">{{ vm.vcpus }}</vcpu>
+    {% endblock %}
     <os>
+        {% block os %}
             {% if vm.hvm %}
                 <type arch="x86_64" machine="xenfv">hvm</type>
                 <loader>hvmloader</loader>
@@ -17,9 +20,11 @@
                 <initrd>{{ vm.storage.kernels_dir }}/initramfs</initrd>
                 <cmdline>root=/dev/mapper/dmroot ro nomodeset console=hvc0 rd_NO_PLYMOUTH 3 {{ vm.kernelopts }}</cmdline>
             {% endif %}
+        {% endblock %}
     </os>
 
     <features>
+        {% block features %}
             {% if vm.hvm %}
                 <pae/>
                 <acpi/>
@@ -33,8 +38,10 @@
                     <e820_host state="on"/>
                 </xen>
             {% endif %}
+        {% endblock %}
     </features>
 
+    {% block clock %}
         {% if vm.hvm %}
             {% set timezone = vm.features.check_with_template('timezone', 'localtime').lower() %}
             {% if timezone == 'localtime' %}
@@ -49,11 +56,16 @@
                 <timer name="tsc" mode="native"/>
             </clock>
         {% endif %}
+    {% endblock %}
 
+    {% block on %}
         <on_poweroff>destroy</on_poweroff>
         <on_reboot>destroy</on_reboot>
         <on_crash>destroy</on_crash>
+    {% endblock %}
+
     <devices>
+        {% block devices %}
             {% set i = 0 %}
             {# TODO Allow more volumes out of the box #}
             {% set dd = ['e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p',
@@ -116,7 +128,7 @@
                     <target type="xen" port="0"/>
                 </console>
             {% endif %}
-
+        {% endblock %}
     </devices>
 </domain>