xl: add 'mtu' option to network configuration

This patch adds code to parse a value for MTU from the network configuration
if it is present. The documentation in xl-network-configuration.5.pod is
also modified accordingly.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wl@xen.org>

docs/man/xl-network-configuration.5.pod

@@ -249,3 +249,9 @@ on the underlying netback implementation.
 Specifies the devid manually instead of letting xl choose the lowest index available.
 
 NOTE: This should not be set unless you have a reason to.
+
+=head2 mtu
+
+Specifies the MTU (i.e. the maximum size of an IP payload, exclusing headers). The
+default value is 1500 but, if the VIF is attached to a bridge, it will be set to match
+unless overridden by this parameter.

tools/xl/xl_cmdtable.c

@@ -338,7 +338,7 @@ struct cmd_spec cmd_table[] = {
       "Create a new virtual network device",
       "<Domain> [type=<type>] [mac=<mac>] [bridge=<bridge>] "
       "[ip=<ip>] [script=<script>] [backend=<BackDomain>] [vifname=<name>] "
-      "[rate=<rate>] [model=<model>] [accel=<accel>]",
+      "[rate=<rate>] [model=<model>] [accel=<accel>] [mtu=<mtu>]",
     },
     { "network-list",
       &main_networklist, 0, 0,

tools/xl/xl_parse.c

@@ -563,6 +563,8 @@ int parse_nic_config(libxl_device_nic *nic, XLU_Config **config, char *token)
         fprintf(stderr, "the accel parameter for vifs is currently not supported\n");
     } else if (MATCH_OPTION("devid", token, oparg)) {
         nic->devid = parse_ulong(oparg);
+    } else if (MATCH_OPTION("mtu", token, oparg)) {
+        nic->mtu = parse_ulong(oparg);
     } else {
         fprintf(stderr, "unrecognized argument `%s'\n", token);
         return 1;