linux-kernel - Re: [PATCH 3/3] x86, cpu: Change NOP selection for certain Intel CPUs

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Message-ID: <1303169674.7181.74.camel@gandalf.stny.rr.com>
Date:	Mon, 18 Apr 2011 19:34:34 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, Jason Baron <jbaron@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>, tglx@...utronix.de,
	mingo@...e.hu, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 3/3] x86, cpu: Change NOP selection for certain Intel
 CPUs

On Mon, 2011-04-18 at 15:36 -0700, H. Peter Anvin wrote:
> Due to a decoder implementation quirk, some specific Intel CPUs
> actually perform better with the "k8_nops" than with the
> SDM-recommended NOPs.  For runtime-selected NOPs, if we detect those
> specific CPUs then use the k8_nops instead of the ones we would
> normally use.
> 
> Signed-off-by: H. Peter Anvin <hpa@...ux.intel.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Jason Baron <jbaron@...hat.com>
> ---
>  arch/x86/kernel/alternative.c |    9 ++++++++-
>  1 files changed, 8 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 2ca3f65..34bf87e 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -190,7 +190,14 @@ void __init arch_init_ideal_nops(void)
>  {
>  	switch (boot_cpu_data.x86_vendor) {
>  	case X86_VENDOR_INTEL:

Please add:

/*
 * Due to a decoder implementation quirk, some specific Intel CPUs
 * actually perform better with the "k8_nops" than with the
 * SDM-recommended NOPs.  For runtime-selected NOPs, if we detect those
 * specific CPUs then use the k8_nops instead of the ones we would
 * normally use.
 */

I tend to read comments by the code more than change logs that added the
code.

Thanks,

-- Steve

> -		if (boot_cpu_has(X86_FEATURE_NOPL)) {
> +		if (boot_cpu_data.x86 == 6 &&
> +		    boot_cpu_data.x86_model >= 0x0f &&
> +		    boot_cpu_data.x86_model != 0x1c &&
> +		    boot_cpu_data.x86_model != 0x26 &&
> +		    boot_cpu_data.x86_model != 0x27 &&
> +		    boot_cpu_data.x86_model < 0x30) {
> +			ideal_nops = k8_nops;
> +		} else if (boot_cpu_has(X86_FEATURE_NOPL)) {
>  			   ideal_nops = p6_nops;
>  		} else {
>  #ifdef CONFIG_X86_64


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