linux-kernel - Re: [PATCH] kmemcheck: disable fast string operations on P4 CPUs

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Date:	Sat, 21 Feb 2009 18:21:13 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Vegard Nossum <vegard.nossum@...il.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] kmemcheck: disable fast string operations on P4 CPUs

Vegard Nossum wrote:
> This patch may allow us to remove the REP emulation code from
> kmemcheck.
> 
> Signed-off-by: Vegard Nossum <vegard.nossum@...il.com>

Looks good to me!

Acked-by: Pekka Enberg <penberg@...helsinki.fi>

> +#ifdef CONFIG_KMEMCHECK
> +	/*
> +	 * P4s have a "fast strings" feature which causes single-
> +	 * stepping REP instructions to only generate a #DB on
> +	 * cache-line boundaries.
> +	 *
> +	 * Ingo Molnar reported a Pentium D (model 6) and a Xeon
> +	 * (model 2) with the same problem.
> +	 */

Minor nit: I'd move the latter part of the comment to the changelog.

> +	if (c->x86 == 15) {
> +		u64 misc_enable;
> +
> +		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
> +
> +		if (misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING) {
> +			printk(KERN_INFO "kmemcheck: Disabling fast string operations\n");
> +
> +			misc_enable &= ~MSR_IA32_MISC_ENABLE_FAST_STRING;
> +			wrmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
> +		}
> +	}
> +#endif
>  }
>  
>  #ifdef CONFIG_X86_32

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