docs: Add kbase page for Secure Boot

Provide simple recipes for the most common high-level tasks.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

docs/kbase/index.rst

@@ -61,6 +61,9 @@ Usage
 `Snapshots <snapshots.html>`__
     Details about snapshotting a VM
 
+`Secure Boot <secureboot.html>`__
+    Enable and disable the Secure Boot feature
+
 
 Debugging
 ---------

docs/kbase/meson.build

@@ -15,6 +15,7 @@ docs_kbase_files = [
   'qemu-passthrough-security',
   'rpm-deployment',
   's390_protected_virt',
+  'secureboot',
   'secureusage',
   'snapshots',
   'systemtap',

docs/kbase/secureboot.rst

@@ -0,0 +1,102 @@
+===========
+Secure Boot
+===========
+
+.. contents::
+
+Quick configuration
+===================
+
+If you have libvirt 8.6.0 or newer, when creating a new VM you can
+ask for Secure Boot to be enabled with
+
+::
+
+  <os firmware='efi'>
+    <firmware>
+      <feature enabled='yes' name='enrolled-keys'/>
+    </firmware>
+  </os>
+
+and for it to be disabled with
+
+::
+
+  <os firmware='efi'>
+    <firmware>
+      <feature enabled='no' name='enrolled-keys'/>
+    </firmware>
+  </os>
+
+These configuration will cause unsigned guest operating systems to
+be rejected and allowed respectively.
+
+
+Older libvirt versions
+======================
+
+If your libvirt version is older than 8.6.0 but newer than 7.2.0,
+then enabling Secure Boot requires a slightly more verbose XML
+snippet:
+
+::
+
+  <os firmware='efi'>
+    <loader secure='yes'/>
+    <firmware>
+      <feature enabled='yes' name='enrolled-keys'/>
+    </firmware>
+  </os>
+
+Versions older than 7.2.0 require manually providing all information
+about the firmware and are not covered here. Plese refer to `the
+relevant documentation
+<../formatdomain.html#operating-system-booting>`__ for details.
+
+
+Changing an existing VM
+=======================
+
+Once the VM has been created, updating the XML configuration as
+described above is **not** enough to change the Secure Boot status:
+the NVRAM file associated with the VM has to be regenerated from its
+template as well.
+
+In order to do that, update the XML and then start the VM with
+
+::
+
+  $ virsh start $vm --reset-nvram
+
+This option is only available starting with libvirt 8.1.0, so if your
+version of libvirt is older than that you will have to delete the
+NVRAM file manually before starting the VM.
+
+Most guest operating systems will be able to cope with the NVRAM file
+being reinitialized, but in some cases the VM will be unable to boot
+after the change.
+
+
+Additional information
+======================
+
+There are two parts to enabling Secure Boot: the firmware supporting
+the feature, and it being active.
+
+Most host operating systems ship a build of EDKII (the open source
+EFI implementation used for QEMU VMs) that supports the Secure Boot
+feature, but simply using such a build will not result in unsigned
+guest operating systems being rejected: for that to happen, keys that
+can be used to validate the operating system signature need to be
+provided as well.
+
+Asking for the ``enrolled-keys`` firmware feature to be enabled will
+cause libvirt to initialize the NVRAM file associated with the VM
+from a template that contains a suitable set of keys. These keys
+being present will cause the firmware to enforce the Secure Boot
+signing requirements.
+
+The opposite configuration, where the feature is explicitly disabled,
+will result in no keys being present in the NVRAM file. Unable to
+verify signatures, the firmware will allow even unsigned operating
+systems to run.