[PATCH v7] virtio-net: add Max MTU configuration field

[Aaron Conole <aconole@redhat.com>]

Sometimes it is essential for a host to be able to configure MTU
on guest's NICs to a value other than the assumed 1500 byte value.

The change adds a new field to configuration area of network
devices. It will be used to pass a maximum MTU from the device to
the driver. This will be used by the driver as a maximum value for
packet sizes during transmission, without transmit segment offloading.

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

Signed-off-by: Aaron Conole <aconole@redhat.com>
Cc: Victor Kaplansky <victork@redhat.com>
---
Previous thread:
https://lists.oasis-open.org/archives/virtio-dev/201603/msg00001.html

Note: should this proposal be accepted and approved, one or more
      claims disclosed to the TC admin and listed on the Virtio TC
      IPR page https://www.oasis-open.org/committees/virtio/ipr.php
      might become Essential Claims.

 content.tex | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/content.tex b/content.tex
index 4b45678..4dc9beb 100644
--- a/content.tex
+++ b/content.tex
@@ -3049,6 +3049,14 @@ features.
 \item[VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2)] Control channel offloads
         reconfiguration support.
 
+\item[VIRTIO_NET_F_MTU(3)] Maximum negotiated MTU is supported. If
+    offered by the device, device advises driver about the value of
+    MTU to be used. If negotiated, the driver uses \field{mtu} as
+    the maximum MTU value supplied to the operating system.
+
+    Note: many operating systems override the MTU value provided by the
+    driver.
+
 \item[VIRTIO_NET_F_MAC (5)] Device has given MAC address.
 
 \item[VIRTIO_NET_F_GUEST_TSO4 (7)] Driver can receive TSOv4.
@@ -3140,11 +3148,16 @@ of each of transmit and receive virtqueues (receiveq1\ldots receiveqN
 and transmitq1\ldots transmitqN respectively) that can be configured once VIRTIO_NET_F_MQ
 is negotiated.
 
+The following driver-read-only field, \field{mtu} only exists if
+VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the driver to
+use.
+
 \begin{lstlisting}
 struct virtio_net_config {
         u8 mac[6];
         le16 status;
         le16 max_virtqueue_pairs;
+        le16 mtu;
 };
 \end{lstlisting}
 
@@ -3153,6 +3166,19 @@ struct virtio_net_config {
 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.
+
+The device MUST NOT modify \field{mtu} once it has been set.
+
+The device MUST NOT pass received packets that exceed \field{mtu} size
+with gso type NONE or ECN.
+
+The device MUST forward transmitted packets of up to MTU size with
+gso type NONE or ECN, without fragmentation.
+
 \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.
@@ -3165,6 +3191,16 @@ If the driver does not negotiate the VIRTIO_NET_F_STATUS feature, it SHOULD
 assume the link is active, otherwise it SHOULD read the link status from
 the bottom bit of \field{status}.
 
+A driver SHOULD supply enough receive buffers to be able to receive at least
+one receive packet with gso type NONE or ECN.
+
+A driver SHOULD negotiate VIRTIO_NET_F_MTU if the device offers it.
+
+A driver MUST honor \field{mtu} if it negotiates VIRTIO_NET_F_MTU.
+
+A driver MUST NOT transmit packets of size exceeding the value of \field{mtu}
+with gso type NONE or ECN, if it negotiates VIRTIO_NET_F_MTU.
+
 \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
-- 
2.5.5