linux-kernel - Re: [PATCH 2/9] x86/kernel/cpu/intel.c: Initialize Enhanced REP MOVSB/STOSBenhanced

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Date:	Tue, 17 May 2011 20:47:23 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Asit K Mallick <asit.k.mallick@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9] x86/kernel/cpu/intel.c: Initialize Enhanced REP MOVSB/STOSBenhanced

On 05/17/2011 07:46 PM, Andi Kleen wrote:
>> From: Fenghua Yu <fenghua.yu@...el.com>
>>
>> If kernel intends to use enhanced REP MOVSB/STOSB, it must ensure
>> IA32_MISC_ENABLE.Fast_String_Enable (bit 0) is set and CPUID.(EAX=07H,
>> ECX=0H):
>> EBX[bit 9] also reports 1.
> 
> I suspect the check at this place is not too useful because it will
> only work for the BSP. For all others it's too late -- the patching
> has already happened.
> 
> So either this is a problem and then it should be checked on all CPUs.
> Or maybe not at all.
> 
> The problem is that the alternative patching currently relies on being
> run early with no other CPUs. It has no race protections, support
> for cross modification etc.
> 
> While it would be possible to fix that it would be quite complicated
> I bet.
> 
> So I think it's better to just remove it unless it's a real problem
> in the field.
> 

The reason for having it is that if the BIOS has a chicken switch that
disables FAST_STRING (and it's the BIOS' responsibility to do it on all
CPUs) then this will make the kernel honor this.

	-hpa
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