launch-libvirt: add virtio-net via the standard <interface> element

Starting with version 3.8.0, libvirt allows us to specify the network
address and network mask (as prefix) for SLIRP directly via the
<interface> element in the domain XML:
<https://libvirt.org/formatdomain.html#userspace-slirp-stack>. This means
we don't need the <qemu:commandline> hack for virtio-net on such versions.

Restrict the hack in construct_libvirt_xml_qemu_cmdline() to
libvirt<3.8.0, and generate the proper <interface> element in
construct_libvirt_xml_devices() on libvirt>=3.8.0.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2034160
Suggested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20211223103701.12702-4-lersek@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>

lib/guestfs-internal.h

@@ -173,7 +173,8 @@ cleanup_mutex_unlock (pthread_mutex_t **ptr)
 #endif
 
 /* Place the virtio-net controller in slot 0x1e on the root bus, on normal
- * hardware with PCI. Refer to RHBZ#2034160.
+ * hardware with PCI. Necessary only before libvirt 3.8.0. Refer to
+ * RHBZ#2034160.
  */
 #ifdef HAVE_LIBVIRT_BACKEND
 #if defined(__arm__) || defined(__s390x__)

lib/launch-libvirt.c

@@ -1413,6 +1413,28 @@ construct_libvirt_xml_devices (guestfs_h *g,
       } end_element ();
     } end_element ();
 
+    /* Virtio-net NIC with SLIRP (= userspace) back-end, if networking is
+     * enabled. Starting with libvirt 3.8.0, we can specify the network address
+     * and prefix for SLIRP in the domain XML. Therefore, we can add the NIC
+     * via the standard <interface> element rather than <qemu:commandline>, and
+     * so libvirt can manage the PCI address of the virtio-net NIC like the PCI
+     * addresses of all other devices. Refer to RHBZ#2034160.
+     */
+    if (g->enable_network &&
+        guestfs_int_version_ge (&params->data->libvirt_version, 3, 8, 0)) {
+      start_element ("interface") {
+        attribute ("type", "user");
+        start_element ("model") {
+          attribute ("type", "virtio");
+        } end_element ();
+        start_element ("ip") {
+          attribute ("family", "ipv4");
+          attribute ("address", NETWORK_ADDRESS);
+          attribute ("prefix", NETWORK_PREFIX);
+        } end_element ();
+      } end_element ();
+    }
+
     /* Libvirt adds some devices by default.  Indicate to libvirt
      * that we don't want them.
      */
@@ -1835,9 +1857,10 @@ construct_libvirt_xml_qemu_cmdline (guestfs_h *g,
     } end_element ();
 
     /* Workaround because libvirt user networking cannot specify "net="
-     * parameter.
+     * parameter. Necessary only before libvirt 3.8.0; refer to RHBZ#2034160.
      */
-    if (g->enable_network) {
+    if (g->enable_network &&
+        !guestfs_int_version_ge (&params->data->libvirt_version, 3, 8, 0)) {
       start_element ("qemu:arg") {
         attribute ("value", "-netdev");
       } end_element ();