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Date:	Tue, 10 Nov 2009 21:50:07 +0200
From:	Avi Kivity <>
To:	"H. Peter Anvin" <>
CC:	Alan Cox <>, Willy Tarreau <>,
	Pavel Machek <>,
	Matteo Croce <>,
	Sven-Haegar Koch <>,
	Ingo Molnar <>,
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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