netdev - Re: [PATCH v2 net-next af-packet 1/2] Enhance af-packet to provide (near zero)lossless packet capture functionality.

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Date:	Thu, 07 Jul 2011 00:13:01 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	loke.chetan@...il.com
Cc:	netdev@...r.kernel.org, eric.dumazet@...il.com, joe@...ches.com,
	bhutchings@...arflare.com, shemminger@...tta.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net-next af-packet 1/2] Enhance af-packet to provide
 (near zero)lossless packet capture functionality.

From: chetan loke <loke.chetan@...il.com>
Date: Wed, 6 Jul 2011 17:45:20 -0400

> new format:
> 
> union bd_header_u {
>        /* renamed struct bd_v1 to hdr_v1 */
>        struct hdr_v1 h1;
> } __attribute__ ((__packed__));
> 
> struct block_desc {
>        __u16 version;
>        __u16 offset_to_priv;
>        union bd_header_u hdr;
> } __attribute__ ((__packed__));
> 
> Is this ok with you?

Get rid of __packed__, it's going to kill performance on RISC
platforms.  If you use __packed__, regardless of the actual alignment,
the compiler must assume that each part of the struct "might" be
unaligned.  So on architectures such as sparc where alignment matters,
a word is going to be accessed by a sequence of byte loads/stores.

Do not use packed unless absolutely enforced by a protocol or hardware
data structure, it's evil.
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