Date:	Thu, 20 Aug 2015 16:31:01 -0300
From:	Flavio Leitner <fbl@...close.org>
To:	Victor Kaplansky <victork@...hat.com>
Cc:	virtio-dev@...ts.oasis-open.org, mst@...hat.com,
	jasowang@...hat.com, netdev@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 2/2] virtio-net: add default_mtu configuration field

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 <victork@...hat.com>
> 
>     Signed-off-by: Victor Kaplansky <victork@...hat.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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