linux-kernel - Re: [all better] Re: regression: massive trouble with fpu rework

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Date:	Sat, 27 Jun 2015 10:55:28 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [all better] Re: regression: massive trouble with fpu rework

On Sat, 2015-06-27 at 10:25 +0200, Ingo Molnar wrote:
> * Mike Galbraith <umgwanakikbuti@...il.com> wrote:
> 
> > On Sat, 2015-06-27 at 08:25 +0200, Mike Galbraith wrote:
> > > Hi Ingo,
> > > 
> > > My i7-4790 box is having one hell of a time with this merge window, is
> > > dead in the water.
> > 
> > BIOS setting "Limit CPUID Maximum" upsets new fpu code mightily.
> 
> Ok, that's interesting. Mind explaining it a bit more verbosely - which setting is 
> causing what?

That BIOS setting is annotated with the helpful text "Disabled for
Windows XP".  It makes box say interesting things during boot, like...

        x86/fpu: XSTATE_CPUID missing!


..or with HEAD, it triggers warning..

        if (boot_cpu_data.cpuid_level < XSTATE_CPUID) {
                WARN_ON_FPU(1);
                return;
        }

..and all kinds of bad juju follows.  I have no idea what the thing does
beyond what I can interpolate from the word 'limit'.

> This would suggest sensitivity on CPUID details, i.e. that doing 
> fpu__init_system() before other CPU init sequences is causing the bug.
> 
> Does the patch below perhaps make a difference? (I'd suggest to apply it _without_ 
> the other patch I sent.)

Yup, that made it not care about the BIOS setting.. again.

> Thanks,
> 
> 	Ingo
> 
>  arch/x86/kernel/cpu/common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 9fc5e3d9d9c8..922c5e0cea4c 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -742,7 +742,6 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>  	cpu_detect(c);
>  	get_cpu_vendor(c);
>  	get_cpu_cap(c);
> -	fpu__init_system(c);
>  
>  	if (this_cpu->c_early_init)
>  		this_cpu->c_early_init(c);
> @@ -754,6 +753,7 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>  		this_cpu->c_bsp_init(c);
>  
>  	setup_force_cpu_cap(X86_FEATURE_ALWAYS);
> +	fpu__init_system(c);
>  }
>  
>  void __init early_cpu_init(void)


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