On Sun, Aug 16, 2015 at 04:42:25PM +0300, Victor Kaplansky 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
> 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.
>
> Added conformance statements for a device and a driver.
>
> Signed-off-by: Victor Kaplansky <vict...@redhat.com>
>
> Signed-off-by: Victor Kaplansky <vict...@redhat.com>
> ---
> content.tex | 25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
> diff --git a/content.tex b/content.tex
> index 342183b..439d005 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -3078,6 +3078,12 @@ features.
>
> \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
> channel.
> +
> +\item[VIRTIO_NET_F_DEFAULT_MTU(24)] Default MTU is supported. If
> + offered by the device, device advises driver about initial MTU to
> + be used. If negotiated, the driver uses \field{default_mtu} as
> + an initial value and reports MTU changes to the device.
> +
> \end{description}
>
> \subsubsection{Feature bit requirements}\label{sec:Device Types / Network
> Device / Feature bits / Feature bit requirements}
> @@ -3128,6 +3134,7 @@ struct virtio_net_config {
> u8 mac[6];
> le16 status;
> le16 max_virtqueue_pairs;
> + le16 default_mtu;
> };
> \end{lstlisting}
>
> @@ -3158,6 +3165,15 @@ by the driver after negotiation.
> \field{max_virtqueue_pairs} is valid only if VIRTIO_NET_F_MQ is
> set and can be read by the driver.
>
> +\item [\field{default_mtu}] is a hint to the driver set by the
> + device. It is valid during feature negotiation only if
> + VIRTIO_NET_F_DEFAULT_MTU is offered and holds the initial value
> + of MTU to be used by the driver. If VIRTIO_NET_F_DEFAULT_MTU is
> + negotiated, the driver uses the \field{default_mtu} as an initial
> + value, and also reports MTU changes to the device by writes to
> + \field{default_mtu}. Such reporting can be used for debugging,
As already said, it's better to change to 'mtu' since changes can
be reported back by writing to the field.
fbl
> + or it can be used for tunning MTU along the network.
> +
> \end{description}
>
> \devicenormative{\subsubsection}{Device configuration layout}{Device Types /
> Network Device / Device configuration layout}
> @@ -3165,6 +3181,9 @@ by the driver after negotiation.
> 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{default_mtu} to between 68 and 65535
> +inclusive, if it offers VIRTIO_NET_F_DEFAULT_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.
> @@ -3177,6 +3196,12 @@ 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_DEFAULT_MTU if the device
> +offers it. If the driver negotiates the VIRTIO_NET_F_DEFAULT_MTU
> +feature, the driver MUST use \field{default_mtu} as an initial value
> +for MTU and the driver MUST report the value of MTU to
> +\field{default_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
> --
> --Victor
> --
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