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

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Date:	Sun, 04 May 2008 22:24:41 +0200
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	"H. Peter Anvin" <hpa@...or.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...

H. Peter Anvin wrote:
> Maciej W. Rozycki wrote:
>> On Sat, 3 May 2008, H. Peter Anvin wrote:
>>
>>> Intel explicitly documents "all processors with family 6 or F."
>>
>>  It depends (as usually with Intel) on what document you are looking 
>> at.  Based on my short research, the instruction has been 
>> retroactively added
>> to the list of supported opcodes.  Even my somewhat dated P4 manual does
>> not list it, never mind its predecessors.  It could have been 
>> accidentally
>> omitted or even buggy in some early members of the P6 family and this
>> could have been the reason for not documenting it from the beginning (the
>> case of FFREEP comes to mind).
>>
> 
> 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

> 
>     -hpa

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