netdev - Re: [PATCH net-next 1/1] vlan: restore offload use on vlan transmit

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Date:	Thu, 6 Dec 2012 23:43:47 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Andrew Gallatin <gallatin@...i.com>
CC:	<davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/1] vlan: restore offload use on vlan transmit

On Thu, 2012-12-06 at 15:54 -0500, Andrew Gallatin wrote:
> This patch copies the vlan device's dev->features to its
> dev->vlan_features, which allows packets to arrive at
> vlan_dev_hard_start_xmit() with their offloads intact.
> 
> When a packet destined for a vlan interface is transmitted, it passes
> through dev_hard_start_xmit() (and netif_skb_features()) twice.  First
> on its way to vlan_dev_hard_start_xmit(), and then again on its way to
> the backing device's xmit handler. If the vlan device does not setup
> its dev->vlan_features, then netif_skb_features() will strip the
> features on the first trip through it (on the way to
> vlan_dev_hard_start_xmit()).
> 
> Signed-off-by: Andrew Gallatin <gallatin@...i.com>
> ---
>  net/8021q/vlan_dev.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
> index 4a6d31a..9d4b3c9 100644
> --- a/net/8021q/vlan_dev.c
> +++ b/net/8021q/vlan_dev.c
> @@ -559,6 +559,7 @@ static int vlan_dev_init(struct net_device *dev)
>  
>  	dev->features |= real_dev->vlan_features | NETIF_F_LLTX;
>  	dev->gso_max_size = real_dev->gso_max_size;
> +	dev->vlan_features = dev->features;

To repeat myself: no, this will result in the features being transferred
to VLAN devices stacked on top of this one (to an arbitrary depth!).

Ben.

>  	/* ipv6 shared card related stuff */
>  	dev->dev_id = real_dev->dev_id;

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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