commit 03ad5670f8a7402e30b288a55d088e87685cd1a1
Author: James Dingwall <james@xxxxxxxxxxxxxx>
Date:   Tue Apr 19 12:45:31 2022 +0100

    The set_mtu() function of xen-network-common.sh currently has this code:
    
            if [ ${type_if} = vif ]
            then
                local dev_=${dev#vif}
                local domid=${dev_%.*}
                local devid=${dev_#*.}
    
                local FRONTEND_PATH="/local/domain/$domid/device/vif/$devid"
    
                xenstore_write "$FRONTEND_PATH/mtu" ${mtu}
            fi
    
    This works fine if the device has its default name but if the xen config
    defines the vifname parameter the FRONTEND_PATH is incorrectly constructed.
    Learn the frontend path by reading the appropriate value from the backend.
    
    Also change use of `...` to $(...) for a consistent style in the script
    and adjust the valid check from `mtu > 0` to `mtu >= 68` per RFC 791.
    
    Signed-off-by: James Dingwall <james@xxxxxxxxxxxxxx>

diff --git a/tools/hotplug/Linux/xen-network-common.sh b/tools/hotplug/Linux/xen-network-common.sh
index 42fa704e8d..9a382c39f4 100644
--- a/tools/hotplug/Linux/xen-network-common.sh
+++ b/tools/hotplug/Linux/xen-network-common.sh
@@ -171,24 +171,20 @@ set_mtu () {
     local mtu=$(xenstore_read_default "$XENBUS_PATH/mtu" "")
     if [ -z "$mtu" ]
     then
-        mtu="`ip link show dev ${bridge}| awk '/mtu/ { print $5 }'`"
+        mtu="$(ip link show dev ${bridge}| awk '/mtu/ { print $5 }')"
         if [ -n "$mtu" ]
         then
             log debug "$bridge MTU is $mtu"
         fi
     fi
-    if [ -n "$mtu" ] && [ "$mtu" -gt 0 ]
+    if [ -n "$mtu" ] && [ "$mtu" -ge 68 ]
     then
         log debug "setting $dev MTU to $mtu"
         ip link set dev ${dev} mtu ${mtu} || :
 
         if [ ${type_if} = vif ]
         then
-            local dev_=${dev#vif}
-            local domid=${dev_%.*}
-            local devid=${dev_#*.}
-
-            local FRONTEND_PATH="/local/domain/$domid/device/vif/$devid"
+            local FRONTEND_PATH="$(xenstore_read "$XENBUS_PATH/frontend")"
 
             xenstore_write "$FRONTEND_PATH/mtu" ${mtu}
         fi