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Date: Sat, 10 Feb 2001 10:09:41 +0100
From: Manfred Spraul <manfred@colorfullife.com>
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To: Doug Ledford <dledford@redhat.com>
CC: Linus Torvalds <torvalds@transmeta.com>, linux-kernel@vger.kernel.org
Subject: Re: [beta patch] SSE copy_page() / clear_page()
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Doug Ledford wrote:
> 
> > I have this strong suspicion that your kernel will lock up in a bad way
> > of you have somebody do something like divide by zero without actually
> > touching a single FP instruction after the divide (so that the error has
> > happened, but has not yet been raised as an exception).
> 
> Or much worse, let the kernel mix-and-match SSE and MMX optimized routines
> without doing full saves of the FPU on SSE routines, which leads to FPU saves
> in MMX routines with kernel data in the SSE registers, which then shows up
> when the app touches those SSE registers and you get use space corruption.  My
> code to handle this type of situation was *very* complex, and I don't think I
> ever got it quite perfectly right without simply imposing a rule that the
> kernel could never use both SSE and MMX instructions on the same CPU.
>

I don't see that problem:
* sse_{copy,clear}_page() restore the sse registers before returning.
* the fpu saves into current->thread.i387.f{,x}save never happen from
interrupts.

How can kernel sse values end up in user space? I'm sure I overlook
something, but what?

--
	Manfred
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