netdev - Re: [PATCH net v2 2/3] geneve: Relax MTU constraints

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Message-ID: <56BA2D6D.6090408@cogentembedded.com>
Date:	Tue, 9 Feb 2016 21:18:21 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	David Wragg <david@...ve.works>, netdev@...r.kernel.org,
	dev@...nvswitch.org
Cc:	Jesse Gross <jesse@...nel.org>, David Miller <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Thomas Graf <tgraf@...g.ch>,
	Roopa Prabhu <roopa@...ulusnetworks.com>
Subject: Re: [PATCH net v2 2/3] geneve: Relax MTU constraints

On 02/09/2016 07:47 PM, David Wragg wrote:

> Allow the MTU of geneve devices to be set to large values, in order to
> exploit underlying networks with larger frame sizes.
>
> GENEVE does not have a fixed encapsulation overhead (an openvswitch
> rule can add variable length options), so there is no relevant maximum
> MTU to enforce.  A maximum of IP_MAX_MTU is used instead.
> Encapsulated packets that are too big for the underlying network will
> get dropped on the floor.
>
> Signed-off-by: David Wragg <david@...ve.works>
> ---
>   drivers/net/geneve.c | 12 +++++++++++-
>   1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/geneve.c b/drivers/net/geneve.c
> index 0b14ac3..05cef11 100644
> --- a/drivers/net/geneve.c
> +++ b/drivers/net/geneve.c
> @@ -1039,6 +1039,16 @@ static netdev_tx_t geneve_xmit(struct sk_buff *skb, struct net_device *dev)
>   	return geneve_xmit_skb(skb, dev, info);
>   }
>
> +static int geneve_change_mtu(struct net_device *dev, int new_mtu)
> +{
> +	/* GENEVE overhead is not fixed, so we can't enforce a more
> +	   precise max MTU. */

    The networking code formats comments:

/* Like
  * this.
  */

> +	if (new_mtu < 68 || new_mtu > IP_MAX_MTU)
> +		return -EINVAL;
> +	dev->mtu = new_mtu;
> +	return 0;
> +}
> +
>   static int geneve_fill_metadata_dst(struct net_device *dev, struct sk_buff *skb)
>   {
>   	struct ip_tunnel_info *info = skb_tunnel_info(skb);
[...]

MBR, Sergei