netdev - Re: [PATCH v2 net-next] net: filter: export pkt_type

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Date:	Thu, 04 Sep 2014 06:09:14 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Denis Kirjanov <kirjanov@...il.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Denis Kirjanov <kda@...ux-powerpc.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Markos Chandras <markos.chandras@...tec.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH v2 net-next] net: filter: export pkt_type_offset() helper

On Thu, 2014-09-04 at 13:50 +0200, Hannes Frederic Sowa wrote:

> 
> Denis, I thought about something like this, you can incorperate this in
> your patch if you can give it a test and check for other architectures,
> thanks!
> 


> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 02529fc..87b86aa 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -548,6 +548,10 @@ struct sk_buff {
>  				ip_summed:2,
>  				nohdr:1,
>  				nfctinfo:3;
> +	/* __pkt_type_offset marks the offset of the bitfield pkt_type
> +	 * contains, so bpf can relatively address it
> +	 */
> +	int			__pkt_type_offset[0];

Why is it 'int', while the following uses __u8 ?

This means pkt_type has to stay at this bit offset, and nothing will
detect at compile time or execution time if someone did a reorg or added
a new field (it seems to be the trend lately)

__u8                          bar:4,
                              pkt_type:3,
...




>  	__u8			pkt_type:3,
>  				fclone:2,
>  				ipvs_property:1,
> @@ -647,6 +651,17 @@ struct sk_buff {
>  #define SKB_ALLOC_FCLONE	0x01
>  #define SKB_ALLOC_RX		0x02
>  

> +#ifdef __BIG_ENDIAN_BITFIELD
> +#define PKT_TYPE_MAX   (7 << 5)
> +#else
> +#define PKT_TYPE_MAX   7
> +#endif

It had a sense right before static int pkt_type_offset(void), but here,
a casual reader might be lost.

I am saying this because a reorder of the bit fields is probably needed
to speedup __copy_skb_header() : Grouping together bit fields could
allow optimized word copies instead of bit field manipulations.



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