[PATCH] virtio-net: add mtu configuration field

[Aaron Conole <aconole@redhat.com>]

Sometimes it is essential for libvirt to be able to configure MTU
on guest's NICs to a value different from 1500.

The change adds a new field to configuration area of network
devices. It will be used to pass initial MTU from the device to
the driver, and to pass modified MTU from driver to the device
when a new MTU is assigned by the guest OS.

In addition, in order to support backward and forward
compatibility, we introduce a new feature bit called
VIRTIO_NET_F_DEFAULT_MTU.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Cc: Victor Kaplansky <victork@redhat.com>

---

This is an attempt at continuing the work done by Victor Kaplansky on
mtu negiotiation for virtio-net devices. It attempts to pick up from
https://lists.oasis-open.org/archives/virtio-dev/201508/msg00007.html
and is just a minor blurb from the first patch along with the 2nd patch
from the series, and some of the feedback integrated.

 trunk/content.tex | 20 +++++++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/trunk/content.tex b/trunk/content.tex
--- a/trunk/content.tex	(revision 551)
+++ b/trunk/content.tex	(working copy)
@@ -3078,6 +3078,11 @@
 
 \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
     channel.
+
+\item[VIRTIO_NET_F_MTU(24)] Negotiated MTU is supported. If
+    offered by the device, device advises driver about initial
+    MTU to be used. If negotiated, the driver uses \field{mtu} as
+    an initial value.
 \end{description}
 
 \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device / Feature bits / Feature bit requirements}
@@ -3132,11 +3137,15 @@
 and transmitq1\ldots transmitqN respectively) that can be configured once VIRTIO_NET_F_MQ
 is negotiated.
 
+The following driver-read-only field, \field{mtu} can be written by the driver
+after negotiation.
+
 \begin{lstlisting}
 struct virtio_net_config {
         u8 mac[6];
         le16 status;
         le16 max_virtqueue_pairs;
+        le16 mtu;
 };
 \end{lstlisting}
 
@@ -3145,6 +3154,11 @@
 The device MUST set \field{max_virtqueue_pairs} to between 1 and 0x8000 inclusive,
 if it offers VIRTIO_NET_F_MQ.
 
+\devicenormative{\subsubsection}{Device configuration layout}{Device Types / Network Device / Device configuration layout}
+
+The device MUST set \field{mtu} to between 68 and 65535 inclusive,
+if it offers VIRTIO_NET_F_MTU.
+
 \drivernormative{\subsubsection}{Device configuration layout}{Device Types / Network Device / Device configuration layout}
 
 A driver SHOULD negotiate VIRTIO_NET_F_MAC if the device offers it.
@@ -3157,6 +3171,12 @@
 assume the link is active, otherwise it SHOULD read the link status from
 the bottom bit of \field{status}.
 
+A driver SHOULD negotiate VIRTIO_NET_F_MTU if the device
+offers it.  If the driver negotiates the VIRTIO_NET_F_MTU
+feature, the driver MAY use \field{mtu} as an initial value
+for MTU and the driver MAY report the value of MTU to
+\field{mtu} when MTU is modified by the guest.
+
 \subsubsection{Legacy Interface: Device configuration layout}\label{sec:Device Types / Network Device / Device configuration layout / Legacy Interface: Device configuration layout}
 \label{sec:Device Types / Block Device / Feature bits / Device configuration layout / Legacy Interface: Device configuration layout}
 When using the legacy interface, transitional devices and drivers
--
2.5.0