linux-kernel - Re: [PATCH/RFC] Math-emu kills the kernel on Athlon64 X2 II -- it's terminally broken

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Date:	Tue, 03 Oct 2006 12:53:34 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Andi Kleen <ak@...e.de>
Cc:	Randy Dunlap <rdunlap@...otime.net>,
	Linus Torvalds <torvalds@...l.org>, akpm <akpm@...l.org>,
	Jesper Juhl <jesper.juhl@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC] Math-emu kills the kernel on Athlon64 X2 II --
	it's terminally broken

Ar Maw, 2006-10-03 am 12:11 +0200, ysgrifennodd Andi Kleen:
> On Tuesday 03 October 2006 05:24, Randy Dunlap wrote:
> 
> Actually I looked at the code more closely. It looks like kernel math
> emulation is much more broken. e.g. kernel_fpu_begin() is missing
> code and lots of other paths in i387 that need to check HAVE_HWFP don't.

That check would be wrong anyway. A kernel built with FPU emulation
boots, runs and uses the hardware FPU code correctly. The problem area
is the X86_FEATURE_foo stuff. I think that comes down to a single thing
- if we have FPU disabled clear X86_FEATURE_FXSR|X86_FEATURE_MMX|
X86_FEATURE_SSE* in the boot cpu features during early option
parsing/setup. Basically the emulated FPU is forgetting to tell the
truth about the fact its a very basic FPU.

Alan

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