Re: Math-emu kills the kernel on Athlon64 X2

[Jesper Juhl]

On 19/09/06, Linus Torvalds <torvalds@xxxxxxxx> wrote:
> On Tue, 19 Sep 2006, Jesper Juhl wrote:
> >
> > If I enable the math emulator in 2.6.18-rc7-git2 (only version I've
> > tried this with) and then boot the kernel with "no387" then I only get
> > as far as lilo's "...Booting the kernel." message and then the system
> > hangs.
>
> I'm wondering if it tries to use the MMX/XMM stuff for memcpy and friends.
>
> I'm also wondering why you'd be doing what you seem to try to be doing in
> the first place ;)
Simply to try and find bugs. If we have a math emulator and it's
selectable for my CPU type, then it should damn well work ;-)

> Basically, "no387" doesn't seem to disable any of the fancier FPU
> features, even though it obviously should. If you ask for math emulation,
> you'll get emulation faults for _all_ of the modern MMX stuff too (which
> we don't do).
Hmm, I guess that could be the problem. The emulator should disable
any stuff which it's not able to handle. I've not actually looked very
much at the emulator code yet, so I didn't realize it didn't disable
what it couldn't handle, but getting it to do that sounds like a
sensible first step.

> It's entirely possible that nobody has ever tested this combination.
That was my thought as well, which is exactely why I chose to try it -
thinking that it might expose some bugs in math-emu or elsewhere.


--
Jesper Juhl <jesper.juhl@xxxxxxxxx>
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