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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Aug 26, 2016 at 01:57:53PM -0400, Aaron Conole wrote:
> 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.

add .. of size \field{mtu}  if it negotiates VIRTIO_NET_F_MTU.

> +
> +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