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

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Date:	Tue, 3 Oct 2006 07:48:22 -0700 (PDT)
From:	Linus Torvalds <torvalds@...l.org>
To:	Andi Kleen <ak@...e.de>
cc:	Randy Dunlap <rdunlap@...otime.net>, 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



On Tue, 3 Oct 2006, Andi Kleen 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.

No it's not.

kernel_fpu_begin() has the _one_ test that matters:

	if (thread->status & TS_USEDFPU) {

since if software emulation is on, nobody will ever have the TS_USEDFPU 
flag set.

> Fixing it properly would be much more work.

No. It's all fixed properly already.

The bug is simply on the newer FXSR paths - marking the FPU emulation 
broken is just stupid. 

		Linus
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