linux-kernel - Re: i686 quirk for AMD Geode

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Message-ID: <4AF9C6AB.8080006@zytor.com>
Date:	Tue, 10 Nov 2009 12:01:47 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Avi Kivity <avi@...hat.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 11:50 AM, Avi Kivity wrote:
>>
>> 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).
> 

I picked the example carefully: SSE3 is a small set of instructions
which probably aren't used very heavily.  In that sense, it has
*exactly* the same properties as CMOV - if you have the source, you're
better off recompiling, but it *might* help you if you happen to only
have a binary.

What I want people to understand is that this is a *huge* rathole, and
it doesn't have any obvious bottom that I can see.

	-hpa
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