Re: [Xen-devel] [PATCH] xen-netback: use netdev_alloc_skb_ip_align

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-04-10 at 00:07 +0100, Stephen Hemminger wrote:
> On Tue, 9 Apr 2013 12:27:31 +0100
> Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 
> > (apologies for the late reply, I've been away)
> > 
> > On Wed, 2013-03-20 at 19:21 +0000, Stephen Hemminger wrote:
> > > Use standard helper function to allocate and align received packet.
> > > Compile tested only
> > > 
> > > Signed-off-by: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
> > > 
> > > 
> > > --- a/drivers/net/xen-netback/netback.c   2013-03-07 18:12:52.825300956 
> > > -0800
> > > +++ b/drivers/net/xen-netback/netback.c   2013-03-20 12:09:09.052034865 
> > > -0700
> > > @@ -1357,8 +1357,8 @@ static unsigned xen_netbk_tx_build_gops(
> > >                       ret < MAX_SKB_FRAGS) ?
> > >                   PKT_PROT_LEN : txreq.size;
> > >  
> > > -         skb = alloc_skb(data_len + NET_SKB_PAD + NET_IP_ALIGN,
> > > -                         GFP_ATOMIC | __GFP_NOWARN);
> > > +         skb = __netdev_alloc_skb_ip_align(vif->dev, data_len,
> > > +                                           GFP_ATOMIC | __GFP_NOWARN);
> > 
> > __netdev_alloc_skb_ip_align accounts for NET_IP_ALIGN but not
> > NET_SKB_PAD, is this aspect of the change intentional? (I'm not really
> > sure how much NET_SKB_PAD is worth in practice).
> 
> __nedev_alloc_skb_ip_align(dev, length, gfp)
>  calls __netdev_alloc_skb(dev, length + NET_IP_ALIGN, gfp)
>    calls __netdev_alloc_skb
> 
> which adds padding here:
> struct sk_buff *__netdev_alloc_skb(struct net_device *dev,
>                                  unsigned int length, gfp_t gfp_mask)
> {
>       struct sk_buff *skb = NULL;
>       unsigned int fragsz = SKB_DATA_ALIGN(length + NET_SKB_PAD) +
>                             SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

Thanks for the explanation.

Acked-by: Ian Campbell <ian.campbell@xxxxxxxxx>

I see this wasn't CCd to netdev -- do you want to resend or would you
like me to fwd to DaveM for you?

Ian.




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