diff mbox series

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

Message ID 20200803124931.2678-4-paul@xen.org (mailing list archive)
State New, archived
Headers show
Series tools: propagate bridge MTU to vif frontends | expand

Commit Message

Paul Durrant Aug. 3, 2020, 12:49 p.m. UTC 
From: Paul Durrant <pdurrant@amazon.com>

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@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: George Dunlap <george.dunlap@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Julien Grall <julien@xen.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Wei Liu <wl@xen.org>

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(+)

Comments

Ian Jackson Aug. 4, 2020, 11:10 a.m. UTC | #1 
Paul Durrant writes ("[PATCH v2 3/4] public/io/netif: specify MTU override node"):
> From: Paul Durrant <pdurrant@amazon.com>
> 
> 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@amazon.com>
> Reviewed-by: Juergen Gross <jgross@suse.com>

Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>

> +/*
> + * 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.
> + */

I find this wording a bit clumsy - a more formal treatment might be
better - but in my reading the overall semantics are correct.

Thanks,X
Ian.
diff mbox series

Patch

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
  * ==========