linux-kernel - Re: [PATCH 1/8] drivers/random: Cache align ip

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Date:	Wed, 16 Mar 2011 10:17:19 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	George Spelvin <linux@...izon.com>
cc:	penberg@...helsinki.fi, herbert@...dor.hengli.com.au,
	mpm@...enic.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] drivers/random: Cache align ip_random better

On Sun, 13 Mar 2011, George Spelvin wrote:

> Cache aligning the secret[] buffer makes copying from it infinitesimally
> more efficient.
> ---
>  drivers/char/random.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 72a4fcb..4bcc4f2 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1417,8 +1417,8 @@ static __u32 twothirdsMD4Transform(__u32 const buf[4], __u32 const in[12])
>  #define HASH_MASK ((1 << HASH_BITS) - 1)
>  
>  static struct keydata {
> -	__u32 count; /* already shifted to the final position */
>  	__u32 secret[12];
> +	__u32 count; /* already shifted to the final position */
>  } ____cacheline_aligned ip_keydata[2];
>  
>  static unsigned int ip_cnt;

I'm intrigued: please educate me.  On what architectures does cache-
aligning a 48-byte buffer (previously offset by 4 bytes) speed up
copying from it, and why?  Does the copying involve 8-byte or 16-byte
instructions that benefit from that alignment, rather than cacheline
alignment?
 
Thanks,
Hugh
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