conf: support configuring mtu size in a virtual network

Example:

  <network>
     ...
     <mtu size='9000'/>
     ...

If mtu is unset, it's assumed that we want the default for whatever is
the underlying transport (usually this is 1500).

This setting isn't yet wired in, so it will have no effect.

This partially resolves: https://bugzilla.redhat.com/1224348

docs/formatnetwork.html.in

@@ -93,6 +93,7 @@
     <pre>
 ...
 &lt;bridge name="virbr0" stp="on" delay="5" macTableManager="libvirt"/&gt;
+&lt;mtu size="9000"/&gt;
 &lt;domain name="example.com" localOnly="no"/&gt;
 &lt;forward mode="nat" dev="eth0"/&gt;
 ...</pre>
@@ -151,9 +152,25 @@
           <span class="since">Since 1.2.11, requires kernel 3.17 or
           newer</span>
         </p>
+      </dd>
 
-
+      <dt><code>mtu</code></dt>
+      <dd>
+        The <code>size</code> attribute of the <code>mtu></code>
+        element specifies the Maximum Transmission Unit (MTU) for the
+        network. <span class="since">Since 3.1.0</span>. In the case
+        of a libvirt-managed network (one with forward mode
+        of <code>nat</code>, <code>route</code>, <code>open</code>, or
+        no <code>forward</code> element (i.e. an isolated network),
+        this will be the MTU assigned to the bridge device when
+        libvirt creates it, and thereafter also assigned to all tap
+        devices created to connect guest interfaces. Network types not
+        specifically mentioned here don't support having an MTU set in
+        the libvirt network config. If mtu size is unspecified, the
+        default setting for the type of device being used is assumed
+        (usually 1500).
       </dd>
+
       <dt><code>domain</code></dt>
       <dd>
         The <code>name</code> attribute on the <code>domain</code>

docs/news.xml

@@ -36,10 +36,11 @@
       </change>
       <change>
         <summary>
-          Introduce MTU to domain &lt;interface/&gt;
+          Introduce MTU to domain &lt;interface/&gt; and &lt;network&gt;
         </summary>
         <description>
-          Allow setting MTU size for some types of domain interface.
+          Allow setting MTU size for some types of domain interface
+          and network.
         </description>
       </change>
     </section>

docs/schemas/network.rng

@@ -79,6 +79,11 @@
           </element>
         </optional>
 
+        <!-- <mtu> element -->
+        <optional>
+          <ref name="mtu"/>
+        </optional>
+
         <!-- <mac> element -->
         <optional>
           <element name="mac">

src/conf/network_conf.c

@@ -2242,6 +2242,17 @@ virNetworkDefParseXML(xmlXPathContextPtr ctxt)
         def->mac_specified = true;
     }
 
+    tmp = virXPathString("string(./mtu/@size)", ctxt);
+    if (tmp) {
+        if (virStrToLong_ui(tmp, NULL, 10, &def->mtu) < 0) {
+            virReportError(VIR_ERR_XML_ERROR,
+                           _("Invalid mtu size '%s' in network '%s'"),
+                           tmp, def->name);
+            goto error;
+        }
+    }
+    VIR_FREE(tmp);
+
     dnsNode = virXPathNode("./dns", ctxt);
     if (dnsNode != NULL &&
         virNetworkDNSDefParseXML(def->name, dnsNode, ctxt, &def->dns) < 0) {
@@ -2435,7 +2446,9 @@ virNetworkDefParseXML(xmlXPathContextPtr ctxt)
     case VIR_NETWORK_FORWARD_BRIDGE:
         if (def->delay || stp) {
             virReportError(VIR_ERR_XML_ERROR,
-                           _("bridge delay/stp options only allowed in route, nat, and isolated mode, not in %s (network '%s')"),
+                           _("bridge delay/stp options only allowed in "
+                             "route, nat, and isolated mode, not in %s "
+                             "(network '%s')"),
                            virNetworkForwardTypeToString(def->forward.type),
                            def->name);
             goto error;
@@ -2454,6 +2467,19 @@ virNetworkDefParseXML(xmlXPathContextPtr ctxt)
 
     VIR_FREE(stp);
 
+    if (def->mtu &&
+        (def->forward.type != VIR_NETWORK_FORWARD_NONE &&
+         def->forward.type != VIR_NETWORK_FORWARD_NAT &&
+         def->forward.type != VIR_NETWORK_FORWARD_ROUTE &&
+         def->forward.type != VIR_NETWORK_FORWARD_OPEN)) {
+        virReportError(VIR_ERR_XML_ERROR,
+                       _("mtu size only allowed in open, route, nat, "
+                         "and isolated mode, not in %s (network '%s')"),
+                       virNetworkForwardTypeToString(def->forward.type),
+                       def->name);
+        goto error;
+    }
+
     /* Extract custom metadata */
     if ((metadataNode = virXPathNode("./metadata[1]", ctxt)) != NULL) {
         def->metadata = xmlCopyNode(metadataNode, 1);
@@ -2964,6 +2990,9 @@ virNetworkDefFormatBuf(virBufferPtr buf,
         virBufferAddLit(buf, "/>\n");
     }
 
+    if (def->mtu)
+        virBufferAsprintf(buf, "<mtu size='%u'/>\n", def->mtu);
+
     if (def->mac_specified) {
         char macaddr[VIR_MAC_STRING_BUFLEN];
         virMacAddrFormat(&def->mac, macaddr);

src/conf/network_conf.h

@@ -240,6 +240,7 @@ struct _virNetworkDef {
     int domainLocalOnly; /* enum virTristateBool: yes disables dns forwarding */
     unsigned long delay;   /* Bridge forward delay (ms) */
     bool stp; /* Spanning tree protocol */
+    unsigned int mtu; /* MTU for bridge, 0 means "default" i.e. unset in config */
     virMacAddr mac; /* mac address of bridge device */
     bool mac_specified;
 

tests/networkxml2xmlin/set-mtu.xml

+<network ipv6='no'>
+  <name>private</name>
+  <uuid>81ff0d90-c91e-6742-64da-4a736edb9a9b</uuid>
+  <bridge name='virbr2'/>
+  <mtu size='9000'/>
+  <mac address='52:54:00:17:3F:37'/>
+  <ip address="192.168.152.1" netmask="255.255.255.0">
+    <dhcp>
+      <range start="192.168.152.2" end="192.168.152.254"/>
+    </dhcp>
+  </ip>
+</network>

tests/networkxml2xmlout/set-mtu.xml

+<network>
+  <name>private</name>
+  <uuid>81ff0d90-c91e-6742-64da-4a736edb9a9b</uuid>
+  <bridge name='virbr2' stp='on' delay='0'/>
+  <mtu size='9000'/>
+  <mac address='52:54:00:17:3f:37'/>
+  <ip address='192.168.152.1' netmask='255.255.255.0'>
+    <dhcp>
+      <range start='192.168.152.2' end='192.168.152.254'/>
+    </dhcp>
+  </ip>
+</network>

tests/networkxml2xmltest.c

@@ -158,6 +158,7 @@ mymain(void)
     DO_TEST_PARSE_ERROR("hostdev-duplicate");
     DO_TEST_PARSE_ERROR("passthrough-duplicate");
     DO_TEST("metadata");
+    DO_TEST("set-mtu");
 
     return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
 }