From c7c6de0cb9b00efc203790743ef8c37ba113d3ee Mon Sep 17 00:00:00 2001
From: Paul Durrant <pdurrant@amazon.com>
Date: Tue, 11 Aug 2020 09:02:00 +0100
Subject: [PATCH] tools/hotplug/Linux: modify set_mtu() to optionally use a
 configured value...

...and also inform the frontend.

The set_mtu() function in xen-network-common.sh currently sets the backend
vif MTU to match that of the bridge.

A prior patch added code into libxl such that a tools-configured 'mtu' value
may be present in the xenstore backend area. If the node is present in
xenstore then it should be authoritative. Hence set_mtu() is modified to only
read the MTU of the bridge if it is not present.

The function is also modified to write whatever value it applies to the
backend vif into the xenstore frontend area where is may then be used to
configure the frontend network stack.

NOTE: There is also a small modification replacing '$mtu' with '${mtu}'
      for style consistency.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wl@xen.org>
---
 tools/hotplug/Linux/vif-bridge            |  2 +-
 tools/hotplug/Linux/xen-network-common.sh | 28 +++++++++++++++++++++--
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/tools/hotplug/Linux/vif-bridge b/tools/hotplug/Linux/vif-bridge
index e1d7c49788..b99cc82a21 100644
--- a/tools/hotplug/Linux/vif-bridge
+++ b/tools/hotplug/Linux/vif-bridge
@@ -81,7 +81,7 @@ case "$command" in
         ;&
     online)
         setup_virtual_bridge_port "$dev"
-        set_mtu "$bridge" "$dev"
+        set_mtu "$bridge" "$dev" "$type_if"
         add_to_bridge "$bridge" "$dev"
         ;;
     remove)
diff --git a/tools/hotplug/Linux/xen-network-common.sh b/tools/hotplug/Linux/xen-network-common.sh
index 6a0904361f..82ee70aab4 100644
--- a/tools/hotplug/Linux/xen-network-common.sh
+++ b/tools/hotplug/Linux/xen-network-common.sh
@@ -164,9 +164,33 @@ remove_from_bridge () {
 set_mtu () {
     local bridge=$1
     local dev=$2
-    mtu="`ip link show dev ${bridge}| awk '/mtu/ { print $5 }'`"
+    local type_if=$3
+
+    XENBUS_PATH="${XENBUS_PATH:?}"
+
+    local mtu=$(xenstore_read_default "$XENBUS_PATH/mtu" "")
+    if [ -z "$mtu" ]
+    then
+        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 ]
     then
-            ip link set dev ${dev} mtu $mtu || :
+        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"
+
+            xenstore_write "$FRONTEND_PATH/mtu" ${mtu}
+        fi
     fi
 }
-- 
2.39.5