linux-kernel - Re: [PATCH] x86/amd: disable way access filter on affected CPUs

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Message-ID: <20121024104609.GA13010@gmail.com>
Date:	Wed, 24 Oct 2012 12:46:09 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andre Przywara <andre.przywara@....com>
Cc:	hpa@...or.com, tglx@...utronix.de, mingo@...e.hu,
	linux-kernel@...r.kernel.org, andreas.herrmann3@....com
Subject: Re: [PATCH] x86/amd: disable way access filter on affected CPUs


* Andre Przywara <andre.przywara@....com> wrote:

> The WAF may hurt the performance of some workloads, caused by
> aliasing issues in the L1 cache.
> Disable it on the affected CPUs.
> 
> Signed-off-by: Andre Przywara <andre.przywara@....com>
> ---
>  arch/x86/kernel/cpu/amd.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index f7e98a2..1b7d165 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -631,6 +631,20 @@ static void __cpuinit init_amd(struct cpuinfo_x86 *c)
>  		}
>  	}
>  
> +	/*
> +	 * The way access filter has a performance penalty on some workloads.
> +	 * Disable it on the affected CPUs.
> +	 */
> +	if ((c->x86 == 0x15) &&
> +	    (c->x86_model >= 0x02) && (c->x86_model < 0x20)) {
> +		u64 val;
> +
> +		if (!rdmsrl_safe(0xc0011021, &val) && !(val & 0x1E)) {
> +			val |= 0x1E;
> +			wrmsrl_safe(0xc0011021, val);
> +		}
> +	}

Would be nice to hear more about the background of this change, 
about the amount of 'penalty' and the nature of the workloads. 
Also, it would be useful to know how the [0x02..0x1f] model 
range was chosen.

Thanks,

	Ingo
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