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

[Yan Vugenfirer <yvugenfi@redhat.com>]

> On 11 Dec 2015, at 18:49, Aaron Conole <aconole@redhat.com> 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.

Sorry for missing the initial patches. Here are my 2 cents for what will happen on Windows. The driver will start with MTU configuration set by the OS, 
in this case it will immediately try to set this MTU value to the device. The driver has no means to report the OS device's MTU. Therefore if we want 
the initial value to have a meaning on Windows, the configuration should be done by the means of qemu-ga and not through device configuration.

Best regards,
Yan.

> 
> 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>
> 
> ---
> 
> 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.
> 
> trunk/content.tex | 20 +++++++++++++++++++++++++
> 1 file changed, 20 insertions(+)
> 
> diff --git a/trunk/content.tex b/trunk/content.tex
> --- a/trunk/content.tex	(revision 551)
> +++ b/trunk/content.tex	(working copy)
> @@ -3078,6 +3078,11 @@
> 
> \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 +3137,15 @@
> and transmitq1\ldots transmitqN respectively) that can be configured once VIRTIO_NET_F_MQ
> is negotiated.
> 
> +The following driver-read-only field, \field{mtu} can be written by the driver
> +after negotiation.
> +
> \begin{lstlisting}
> struct virtio_net_config {
>         u8 mac[6];
>         le16 status;
>         le16 max_virtqueue_pairs;
> +        le16 mtu;
> };
> \end{lstlisting}
> 
> @@ -3145,6 +3154,11 @@
> 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.
> +
> \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 +3171,12 @@
> 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 MAY use \field{mtu} as an initial value
> +for MTU and the driver MAY report the value of MTU to
> +\field{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
> --
> 2.5.0
> 
> ---------------------------------------------------------------------
> To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org
> For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
>