netdev - Re: [PATCH v4 net-next] MPLS: Use mpls_features to activate software MPLS GSO segmentation

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Date:	Mon, 2 Jun 2014 17:21:45 +0100
From:	Thomas Graf <tgraf@...g.ch>
To:	Simon Horman <horms@...ge.net.au>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Jesse Gross <jesse@...ira.com>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>, dev@...nvswitch.org
Subject: Re: [PATCH v4 net-next] MPLS: Use mpls_features to activate software
 MPLS GSO segmentation

On 06/02/14 at 01:43pm, Simon Horman wrote:
> +#ifdef CONFIG_NET_MPLS_GSO
> +static netdev_features_t net_mpls_features(struct sk_buff *skb,
> +					   struct net_device *dev,
> +					   netdev_features_t features)
> +{
> +	/* There is no support for MPLS LRO. So the only way that
> +	 * an MPLS skb could require GSO segmentation is if it
> +	 * was received as a non-MPLS skb and then became an MPLS skb.
> +	 * This may be effected by Open vSwitch in which case the
> +	 * mac_len will non-zero and not equal to skb_network_offset
> +	 * as the former indicates the end of L2 while the latter indicates
> +	 * the beginning of L3 and there is a gap between them occupied
> +	 * by the MPLS label stack.
> +	 *
> +	 * Thus it is possible to avoid traversing any VLAN tags that are
> +	 * present to determine if the ethtype is MPLS. Instead the
> +	 * inequality of mac_len and skb_network_offset are used to
> +	 * determine if a packet is MPLS for the purpose of determining
> +	 * offload features.
> +	 */
> +	if (skb->mac_len && skb->mac_len != skb_network_offset(skb))
> +		features &= dev->mpls_features;
> +	return features;
> +}

Could you elaborate a bit on the safety of this? What about
GRE GSO which sets mac_len to the inner network offset?
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