[PATCH v2 3/4] public/io/netif: specify MTU override node

[Paul Durrant <paul@xxxxxxx>]

From: Paul Durrant <pdurrant@xxxxxxxxxx>

There is currently no documentation to state what MTU a frontend should
adertise to its network stack. It has however long been assumed that the
default value of 1500 is correct.

This patch specifies a mechanism to allow the tools to set the MTU via a
xenstore node in the frontend area and states that the absence of that node
means the frontend should assume an MTU of 1500 octets.

NOTE: The Windows PV frontend has used an MTU sampled from the xenstore
      node specified in this patch.

Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Julien Grall <julien@xxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Wei Liu <wl@xxxxxxx>

v2:
 - Add a note in xenstore-paths highlighting the new xenstore node
---
 docs/misc/xenstore-paths.pandoc |  3 +++
 xen/include/public/io/netif.h   | 12 ++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/docs/misc/xenstore-paths.pandoc b/docs/misc/xenstore-paths.pandoc
index 766e8008dc..5cd5c8a3b9 100644
--- a/docs/misc/xenstore-paths.pandoc
+++ b/docs/misc/xenstore-paths.pandoc
@@ -298,6 +298,9 @@ A virtual keyboard device frontend. Described by
 A virtual network device frontend. Described by
 [xen/include/public/io/netif.h][NETIF]
 
+NOTE: ~/device/vif/$DEVID/mtu can be used to inform the frontend of an
+      increased MTU. (The default MTU is 1500 octets).
+
 #### ~/device/vscsi/$DEVID/* []
 
 A virtual scsi device frontend. Described by
diff --git a/xen/include/public/io/netif.h b/xen/include/public/io/netif.h
index 9fcf91a2fe..00dd258712 100644
--- a/xen/include/public/io/netif.h
+++ b/xen/include/public/io/netif.h
@@ -204,6 +204,18 @@
  * present).
  */
 
+/*
+ * MTU
+ * ===
+ *
+ * The toolstack may set a value of MTU for the frontend by setting the
+ * /local/domain/<domid>/device/vif/<vif>/mtu node with the MTU value in
+ * octets. If this node is absent the frontend should assume an MTU value
+ * of 1500 octets. A frontend is also at liberty to ignore this value so
+ * it is only suitable for informing the frontend that a packet payload
+ * >1500 octets is permitted.
+ */
+
 /*
  * Hash types
  * ==========
-- 
2.20.1