Re: [PATCH v2] virtio-net: add mtu configuration field

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

On Fri, Jan 15, 2016 at 04:28:30PM -0500, Aaron Conole wrote:
> 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
> or from device to driver for both host-guest and guest-host
> interaction.
> 
> 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>

Since apparently windows guests can't support
such mtu negotiation in the driver, we'd have
to implement this in a guest agent for these
guests.

It thus seems advantageous to always have it in
a guest agent, so that libvirt can change it
for all guests in the same way, consistently.

Do you agree to the above? If yes, why do we need
a new device field?

> ---
> v1:
> 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.
> 
> v2:
> Rephrase and provide a mechanism for guest->host and host->guest
> communication through a driver read-only and driver write-only field.
> 
>  content.tex | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/content.tex b/content.tex
> index d989d98..b855691 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -3078,6 +3078,11 @@ features.
>  
>  \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 +3137,21 @@ 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 MTU that the device expects
> +the driver to honor.
> +
> +The following driver-write-only field, \field{mtu_requested} only exists if
> +VIRTIO_NET_F_MTU is set. This field will be written by the driver when it wants
> +to request an MTU change from the device.
> +
>  \begin{lstlisting}
>  struct virtio_net_config {
>          u8 mac[6];
>          le16 status;
>          le16 max_virtqueue_pairs;
> +        le16 mtu;
> +        le16 mtu_requested;
>  };
>  \end{lstlisting}
>  
> @@ -3145,6 +3160,18 @@ 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.
> +
> +Any time the device modifies \field{mtu}, it MUST send a configuration change
> +notification.
> +
> +The driver MAY set \field{mtu_requested} to between 68 and 65535 inclusive,
> +if the device offers VIRTIO_NET_F_MTU. Any time the driver modifies
> +\field{mtu_requested}, it MUST also send a configuration change notification.
> +
>  \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 +3184,10 @@ 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 the VIRTIO_NET_F_MTU feature, the driver MUST use \field{mtu}
> +as the value for 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
> -- 
> 2.5.0