linux-kernel - Re: i387/FPU init issues...

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Date:	Sun, 04 May 2008 14:07:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
CC:	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	Thomas Gleixner <tglx@...utronix.de>, jamal <hadi@...erus.ca>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Jan Beulich <jbeulich@...ell.com>
Subject: Re: i387/FPU init issues...

Arjan van de Ven wrote:
>>
>> It has retroactively been added to the documented list for all P6 core 
>> chips - that should mean it works on all of them.  The most common 
>> reason for not documenting something (other than various Pure Evil NDA 
>> schemes) is that it hasn't been properly verified.  However, 
>> verification can be done a posteori.
> 
> the other reason is that certain groups of "unknown" opcodes will act as 
> NOP.
> Which for this purpose is .. exactly the right thing retroactive
> 

Specifically, I believe the P6 added a whole class of instructions which 
would act as NOP's if used on older chips.  It has been used for 
prefetch instructions, etc.  At some point, the 0F 1F /0 group was 
declared to be NOP, and nothing else, for all future.

	-hpa

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