netdev - Re: VLAN driver question

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Date:	Wed, 19 Jun 2013 17:33:55 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Jim Baxter <jim_baxter@...tor.com>
CC:	<netdev@...r.kernel.org>
Subject: Re: VLAN driver question

On Wed, 2013-06-19 at 16:57 +0100, Jim Baxter wrote:
> Thank you Ben, that is very helpful.
> 
> On 18/06/13 15:41, Ben Hutchings wrote:
> > On Mon, 2013-06-17 at 20:12 +0100, Jim Baxter wrote:
> >> I have a network card that has a single flag to indicate if a received
> >> packet contains a vlan packet.
> >>
> >> Can I use this to accelerate the kernels handling of the packet with
> >> something like the following?
> >>
> >> /* Handle received VLAN packets */
> >> if ((ndev->features & NETIF_F_HW_VLAN_CTAG_RX) && ebdp &&
> >> 					(ebdp->cbd_esc &
> >> 					BD_ENET_RX_VLAN)) {
> >> 	/* Push and remove the vlan tag */
> >> 	struct vlan_hdr *vlan_header;
> >> 	u16 vlan_tag;
> >> 	vlan_header = (struct vlan_hdr *) skb->data;
> >> 	vlan_tag = ntohs(vlan_header->h_vlan_TCI);
> >> 	__vlan_hwaccel_put_tag(skb, vlan_tag);
> >> 	skb->len -= VLAN_HLEN;
> >> 	skb->data += VLAN_HLEN;
> >> 	vlan_set_encap_proto(skb, vlan_header);
> > 
> > You have to move the Ethernet addresses forward as well.  Imagine this
> > device is being bridged - the Ethernet header won't be regenerated, so
> > it has to be immediately before the network header.
> > 
> Unless I have misunderstood, the Ethernet header is already copied
> across to the skb by the Driver I am editing with the lines:
> 	skb_reserve(skb, NET_IP_ALIGN);
> 	skb_put(skb, pkt_len - 4);	/* Make room */
> 	skb_copy_to_linear_data(skb, data, pkt_len - 4);
> 	skb->protocol = eth_type_trans(skb, ndev);

Right, but the copy and ether_type_trans() result in an skb like this:

-14 -8  -2   0   2    4
|dst|src|8100|vid|type|payload...|
             ^

After extracting the VLAN tag the skb looks like:

-18 -12 -6   -4  -2   0
|dst|src|8100|vid|type|payload...|
                      ^

If this packet is bridged, the 14 bytes before skb->data will be treated
as the Ethernet header:

0 2   8    10  12   14
|x|src|8100|vid|type|payload...|
^

Then with the VLAN tag inserted;

0 2   8    10  12   14  16   18
|x|src|8100|vid|8100|vid|type|payload...|

> Thinking about it though I should actually use the flag from the driver
> to create an skb of the size (pkt_len - 4 - VLAN_HLEN) and extract the
> vlan header from data so it is not copied to the skb, does this sound
> sensible?
[...]

That would also work.

Ben.

-- 
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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