Date: Wed, 15 Sep 2004 17:35:28 +0200
From: Tonnerre <tonnerre@thundrix.ch>
To: linux-kernel@vger.kernel.org
Subject: Re: [2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float
Message-ID: <20040915153528.GE24818@thundrix.ch>
References: <20040915021418.A1621@natasha.ward.six>
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Salut,

On Wed, Sep 15, 2004 at 02:14:18AM +0600, Denis Zaitsev wrote:
> Why this kernel is always compiled with the FP emulation for x86?
> This is the line from the beginning of arch/i386/Makefile:
>=20
> CFLAGS +=3D -pipe -msoft-float
>=20
> And it's hardcoded, it does not depend on CONFIG_MATH_EMULATION.  So,
> is this just a typo or not?

The problem  is that  the kernel can't  use the  FPU. I think  this is
because  its context  is  not  saved on  context  switch (userland  ->
kernel),  so  we'd end  up  messing up  the  FPU  state, and  userland
applications  would get  silly results  for calculations  with context
switches in between.

Thus  we force gcc  to use  the library  functions for  floating point
arith, and  since we  don't link  against gcc's lib,  FPU users  get a
fancy linker error.

If  you want to  use floating  point arith  inside the  kernel, you're
probably wrong wanting it. If you really need it, you can

a) emulate it using fixed-point math on unsigned long or
b) manually save the FPU state, load your operations into it, operate,
   get the results and restore the FPU state.

I have yet to see someone  who really needs to do floating point maths
inside the kernel.

			    Tonnerre

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