linux-kernel - Re: i686 quirk for AMD Geode

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Message-ID: <4AF9C3EF.6000705@redhat.com>
Date:	Tue, 10 Nov 2009 21:50:07 +0200
From:	Avi Kivity <avi@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>, Willy Tarreau <w@....eu>,
	Pavel Machek <pavel@....cz>,
	Matteo Croce <technoboy85@...il.com>,
	Sven-Haegar Koch <haegar@...net.de>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: i686 quirk for AMD Geode

On 11/10/2009 08:49 PM, H. Peter Anvin wrote:
>
>> Why is using KVM doing it right ? It sounds like its doing it slowly,
>> and hideously memory inefficiently. You are solving an uninteresting
>> general case problem when you just need two tiny fixups (or perhaps 3 if
>> you want to fix up early x86-64 prefetch)
>>      
> Why do we only need "two tiny fixups"?  Where do we draw the line in
> terms of ISA compatibility?  One could easily argue that the Right
> Thing[TM] is to be able to process any optional instruction -- otherwise
> one has a very difficult place to draw a line.
>
> Consider SSE3, for example.  Why should the same concept not apply to
> SSE3 instructions as to CMOV?
>    

Because then user programs would run 20x or more slower than the user 
expects.  Better to terminate early (and teach userspace how to choose 
the instruction subset correctly).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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