Re: [net-next 4/5] netback: set transport header before passing it to kernel

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2013-03-26 at 06:19 +0000, Jason Wang wrote:
> Currently, for the packets receives from netback, before doing header check,
> kernel just reset the transport header in netif_receive_skb() which pretends non
> l4 header. This is suboptimal for precise packet length estimation (introduced
> in 1def9238: net_sched: more precise pkt_len computation) which needs correct l4
> header for gso packets.
> 
> The patch just reuse the header probed by netback for partial checksum packets
> and tries to use skb_flow_dissect() for other cases, if both fail, just pretend
> no l4 header.
> 
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Ian Campbell <ian.campbell@citrix.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/net/xen-netback/netback.c |   12 ++++++++++++
>  1 files changed, 12 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index aa28550..fc8faa7 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -39,6 +39,7 @@
>  #include <linux/udp.h>
>  
>  #include <net/tcp.h>
> +#include <net/flow_keys.h>
>  
>  #include <xen/xen.h>
>  #include <xen/events.h>
> @@ -1184,6 +1185,7 @@ static int checksum_setup(struct xenvif *vif, struct sk_buff *skb)
>  	if (th >= skb_tail_pointer(skb))
>  		goto out;
>  
> +	skb_set_transport_header(skb, 4 * iph->ihl);
>  	skb->csum_start = th - skb->head;

Should the use of th here (and perhaps above) be replaced with
skb_transport_header() too?

>  	switch (iph->protocol) {
>  	case IPPROTO_TCP:
> @@ -1495,6 +1497,7 @@ static void xen_netbk_tx_submit(struct xen_netbk *netbk)
>  
>  		skb->dev      = vif->dev;
>  		skb->protocol = eth_type_trans(skb, skb->dev);
> +		skb_reset_network_header(skb);
>  
>  		if (checksum_setup(vif, skb)) {
>  			netdev_dbg(vif->dev,
> @@ -1503,6 +1506,15 @@ static void xen_netbk_tx_submit(struct xen_netbk *netbk)
>  			continue;
>  		}
>  
> +		if (!skb_transport_header_was_set(skb)) {
> +			struct flow_keys keys;
> +
> +			if (skb_flow_dissect(skb, &keys))
> +				skb_set_transport_header(skb, keys.thoff);
> +			else
> +				skb_reset_transport_header(skb);
> +		}
> +
>  		vif->dev->stats.rx_bytes += skb->len;
>  		vif->dev->stats.rx_packets++;
>