RE: [Xen-devel] [PATCH 2/2] netfront skb padding

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> It appears that when alloc'ing a skb, it is bring padded by 
> an arbitrarily 
> (and excessive) long value.  The value for this padding 
> really only needs to 
> be 24.  24 = 14 for the ethernet header + 2 for the cache 
> alignment + 4 for 
> the CRC + 4 for the VLAN flags.

Given that we're allocating page sized buffers the current situation
doesn't cost us anything.

Infact, what happens if the packet gets encapsulated e.g. by etherip
etc? Is Linux smart enough to be able to put the extra headers on
in-place if there is enough head room?

If so, the current situation is positively better.

Ian

> Tested on Xen 2.0.3 and linux 2.6.10 kernel.
> 
> Signed-off-by: Jon Mason <jdmason@xxxxxxxxxx>
> ---
> 
> --- drivers/xen/netfront/netfront.c.new 2005-02-08 
> 19:09:31.368834000 -0600
> +++ drivers/xen/netfront/netfront.c 2005-02-08 
> 19:09:24.738834000 -0600
> @@ -50,6 +50,8 @@
>  #include <asm-xen/balloon.h>
>  #include <asm/page.h>
>  
> +#define NETFRONT_VERSION "1.1"
> +
>  #ifndef __GFP_NOWARN
>  #define __GFP_NOWARN 0
>  #endif
> @@ -62,8 +64,11 @@
>          skb_shinfo(_skb)->frag_list = NULL;           \
>      } while (0)
>  
> -/* Allow headroom on each rx pkt for Ethernet header, 
> alignment padding, ... */
> -#define RX_HEADROOM 200
> +/* 
> + * Allow headroom on each rx pkt for Ethernet header, 
> alignment padding, 
> + * CRC, and VLAN 
> + */
> +#define RX_HEADROOM ETH_HLEN + NET_IP_ALIGN + 8 
>  
>  /*
>   * If the backend driver is pipelining transmit requests 
> then we can be very
> @@ -1248,7 +1253,8 @@ static int __init netif_init(void)
>      if (xen_start_info.flags & SIF_INITDOMAIN)
>          return 0;
>  
> -    IPRINTK("Initialising virtual ethernet driver.\n");
> +    IPRINTK("Initialising virtual ethernet driver, version %s\n", 
> +      NETFRONT_VERSION);
>      INIT_LIST_HEAD(&dev_list);
>      (void)register_inetaddr_notifier(&notifier_inetdev);
>      netctrl_init();
> 
> 
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