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Subject: Re: [virtio-dev] [PATCH v9] virtio-net: add Max MTU configuration field

On Tue,  6 Sep 2016 10:31:01 -0400
Aaron Conole <aconole@redhat.com> wrote:

<sorry for being late to the party, but I just got around to looking at
this as this is up for vote>

> diff --git a/content.tex b/content.tex
> index 4b45678..b90cbad 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.

I'm wondering: Do we need to distinguish between what the _driver_ does
and what the _guest_ does, generally speaking?

> +
>  \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 +3166,18 @@ 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.
> +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.

Should this rather be relaxed to "after VIRTIO_NET_F_MTU has been
successfully negotiated"? I don't think the driver should assume
anything until after feature negotiation is complete.

> +
> +The device MUST NOT pass received packets that exceed \field{mtu} size
> +with \field{gso_type} NONE or ECN if it offers VIRTIO_NET_F_MTU.

I don't think it should be the offer that is the deciding factor, but
rather the final negotiation. But maybe we can make this "SHOULD NOT"?

> +
> +The device MUST forward transmitted packets of up to MTU size with
> +\field{gso_type} NONE or ECN, and do so without fragmentation, 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.
> @@ -3165,6 +3190,15 @@ 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}.
> +If the device offers VIRTIO_NET_F_MTU, a driver MUST supply enough receive
> +buffers of size \field{mtu} to be able to receive at least one receive
> +packet with \field{gso_type} NONE or ECN.

Again, this should be the final negotiation instead of the offer,
especially as...

> +
> +A driver SHOULD negotiate VIRTIO_NET_F_MTU if the device offers it.

...this is SHOULD and not MUST. The driver should be able to fail
negotiating the feature if it cannot fulfill the condition above.

> +
> +If the device offers VIRTIO_NET_F_MTU, a driver MUST NOT transmit packets of
> +size exceeding the value of \field{mtu} with \field{gso_type} NONE or ECN
> +
>  \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

Again, sorry for being late with comments, but I'll vote with no for
now. I can still change the vote if you can convince me, though.

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