[2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[Denis Zaitsev]

Why this kernel is always compiled with the FP emulation for x86?
This is the line from the beginning of arch/i386/Makefile:

CFLAGS += -pipe -msoft-float

And it's hardcoded, it does not depend on CONFIG_MATH_EMULATION.  So,
is this just a typo or not?

Re: [2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[Alan Cox]

On Maw, 2004-09-14 at 21:14, 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:

To catch incorrect use of FPU code in the kernel

Re: [2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[Brian Gerst]

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:
> 
> CFLAGS += -pipe -msoft-float
> 
> And it's hardcoded, it does not depend on CONFIG_MATH_EMULATION.  So,
> is this just a typo or not?

To trap use of floating point in the kernel (not allowed).  FPU 
emulation is only for userspace.

--
				Brian Gerst

Re: [2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[Arjan van de Ven]

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On Tue, 2004-09-14 at 22:14, 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:
> 
> CFLAGS += -pipe -msoft-float
> 
> And it's hardcoded, it does not depend on CONFIG_MATH_EMULATION.  So,
> is this just a typo or not?


this is on purpose; this way we get an error if someone uses floating
point in the kernel.... which is BAD

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Re: [2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[Dave Jones]

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:
 > 
 > CFLAGS += -pipe -msoft-float
 > 
 > And it's hardcoded, it does not depend on CONFIG_MATH_EMULATION.  So,
 > is this just a typo or not?

It deliberatly causes build failures if folks use fp math in their code.

		Dave

Re: [2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[Matt Mackall]

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:
> 
> CFLAGS += -pipe -msoft-float
> 
> And it's hardcoded, it does not depend on CONFIG_MATH_EMULATION.  So,
> is this just a typo or not?

It catches bogus attempts to use floating point in the kernel by
making them error at link-time.

-- 
Mathematics is the supreme nostalgia of our time.

Re: [2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[Denis Zaitsev]

Thank everybody.

Whould it be better to document the trick?  At least, this will
prevent the others from suspicions.


--- arch/i386/Makefile.orig	2004-08-14 16:55:10.000000000 +0600
+++ arch/i386/Makefile	2004-09-15 03:37:49.000000000 +0600
@@ -20,6 +20,8 @@ OBJCOPYFLAGS	:= -O binary -R .note -R .c
 LDFLAGS_vmlinux :=
 CHECK		:= $(CHECK) -D__i386__=1
 
+# The FP must not be used in the kernel code.  So, the bogus compiler
+# flag is here to catch such an attempts at the link stage.
 CFLAGS += -pipe -msoft-float
 
 # prevent gcc from keeping the stack 16 byte aligned

Re: [2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[Tonnerre]

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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:
> 
> CFLAGS += -pipe -msoft-float
> 
> 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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Re: [2.6.8.1/x86] The kernel is _always_ compiled with -msoft-float

[Ian Campbell]

On Wed, 2004-09-15 at 16:35, Tonnerre wrote:
> 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:
> > 
> > CFLAGS += -pipe -msoft-float
> > 
> > And it's hardcoded, it does not depend on CONFIG_MATH_EMULATION.  So,
> > is this just a typo or not?
[snip]
> 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.

It's a shame that gcc doesn't have -mno-float which could disable
floating point completely and produce a more useful error message than a
missing symbol at link time

I searched for ages just yesterday for the "float" in some kernel code I
acquired recently... To be fair I was grepping for float and it was a
double that was being used so if I'd had my brain turned on I would have
found it quite quickly, but you get the point.

Has anyone ever suggested such a flag to the gcc folks?

Ian.

-- 
Ian Campbell
Current Noise: Megadeth - Pyschotron

Mind your own business, Spock.  I'm sick of your halfbreed interference.