Re: [PATCH] x86: fdiv bug detection fix

[Willy Tarreau <w@1wt.eu>]

On Thu, Jul 31, 2008 at 02:49:20PM -0700, Andrew Morton wrote:
> On Thu, 31 Jul 2008 23:43:44 +0200
> Krzysztof Helt <krzysztof.h1@wp.pl> wrote:
> 
> > From: Krzysztof Helt <krzysztof.h1@wp.pl>
> > 
> > The fdiv detection code writes s32 integer into
> > the boot_cpu_data.fdiv_bug. 
> > However, the boot_cpu_data.fdiv_bug is only char (s8)
> > field so the detection overwrites already set fields for
> > other bugs, e.g. the f00f bug field.
> > 
> > Use local s32 variable to receive result.
> > 
> > Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
> > ---
> > 
> > This is a partial fix to Bugzilla #9928  - fixes wrong 
> > information about the f00f bug (tested) and probably
> > for coma bug (I have no cpu to test this).
> > 
> > diff -urp linux-alsa/arch/x86/kernel/cpu/bugs.c linux-new/arch/x86/kernel/cpu/bugs.c
> > --- linux-alsa/arch/x86/kernel/cpu/bugs.c	2008-07-31 12:40:31.000000000 +0200
> > +++ linux-new/arch/x86/kernel/cpu/bugs.c	2008-07-31 23:24:07.722657435 +0200
> > @@ -50,6 +50,8 @@ static double __initdata y = 3145727.0;
> >   */
> >  static void __init check_fpu(void)
> >  {
> > +	s32 fdiv_bug;
> > +
> >  	if (!boot_cpu_data.hard_math) {
> >  #ifndef CONFIG_MATH_EMULATION
> >  		printk(KERN_EMERG "No coprocessor found and no math emulation present.\n");
> > @@ -74,8 +76,10 @@ static void __init check_fpu(void)
> >  		"fistpl %0\n\t"
> >  		"fwait\n\t"
> >  		"fninit"
> > -		: "=m" (*&boot_cpu_data.fdiv_bug)
> > +		: "=m" (*&fdiv_bug)
> >  		: "m" (*&x), "m" (*&y));
> > +
> > +	boot_cpu_data.fdiv_bug = fdiv_bug;
> >  	if (boot_cpu_data.fdiv_bug)
> >  		printk("Hmm, FPU with FDIV bug.\n");
> >  }
> 
> hm, the code seems to have been that way for quite some time.

Indeed! since boot_cpu_data.fdiv_bug was declared as an int in 2.4, I
think it might have changed when a lot of code was moved from asm/*.h
to arch/*.c.

> I wonder why nobody noticed this before.

For the same reason those bugs were noticed very late in the products'
lifetime I guess : because unless someone actively plays with them on
your machine, you hardly trigger them by accident.

Willy