virtio-net: add Max MTU configuration field

It is helpful for a host to indicate it's MTU to be set on guest NICs
other than the assumed 1500 byte value.  This helps in situations where
the host network is using Jumbo Frames, or aiding in PMTU discovery by
configuring a homogenous network.  It is also helpful for sizing receive
buffers correctly.

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 segmentation offloading.

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

VIRTIO-152

Signed-off-by: Aaron Conole <aconole@redhat.com>
Cc: Victor Kaplansky <victork@redhat.com>
Reviewed-by: Hannes Reiencke <hare@suse.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio/trunk@585 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652

1 files changed, 37 insertions, 0 deletions

diff --git a/content.tex b/content.tex
index 546bef7..222b78e 100644
--- a/content.tex
+++ b/content.tex
@@ -3073,6 +3073,11 @@ features.
 \item[VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2)] Control channel offloads
         reconfiguration support.
 
+\item[VIRTIO_NET_F_MTU(3)] Device maximum MTU reporting is supported. If
+    offered by the device, device advises driver about the value of
+    its maximum MTU. If negotiated, the driver uses \field{mtu} as
+    the maximum MTU value.
+
 \item[VIRTIO_NET_F_MAC (5)] Device has given MAC address.
 
 \item[VIRTIO_NET_F_GUEST_TSO4 (7)] Driver can receive TSOv4.
@@ -3164,11 +3169,16 @@ 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}
 
@@ -3177,6 +3187,23 @@ 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.
 
+The device MUST set \field{mtu} to between 68 and 65535 inclusive,
+if it offers VIRTIO_NET_F_MTU.
+
+The device SHOULD set \field{mtu} to at least 1280, 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} (plus low
+level ethernet header length) size with \field{gso_type} NONE or ECN
+after VIRTIO_NET_F_MTU has been successfully negotiated.
+
+The device MUST forward transmitted packets of up to \field{mtu} (plus low
+level ethernet header length) size with \field{gso_type} NONE or ECN, and do
+so without fragmentation, after VIRTIO_NET_F_MTU has been successfully
+negotiated.
+
 \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.
@@ -3189,6 +3216,16 @@ 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 negotiate VIRTIO_NET_F_MTU if the device offers it.
+
+If the driver negotiates VIRTIO_NET_F_MTU, it MUST supply enough receive
+buffers to receive at least one receive packet of size \field{mtu} (plus low
+level ethernet header length) with \field{gso_type} NONE or ECN.
+
+If the driver negotiates VIRTIO_NET_F_MTU, it MUST NOT transmit packets of
+size exceeding the value of \field{mtu} (plus low level ethernet header length)
+with \field{gso_type} NONE or ECN.
+
 \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