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

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Date:	Mon, 29 Jun 2015 08:40:08 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>, Dave Hansen <dave@...1.net>
Subject: Re: [all better] Re: regression: massive trouble with fpu rework


* Mike Galbraith <umgwanakikbuti@...il.com> wrote:

> > 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.

> > 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);
> >  }

Ok, so could you please move the fpu__init_system() further up and see which 
position is that starts breaking with the BIOS option set?

here's the current, broken layout of the code:

        get_cpu_cap(c);
[0]     fpu__init_system(c);

        if (this_cpu->c_early_init)
                this_cpu->c_early_init(c);

[1]
        c->cpu_index = 0;
[2]
        filter_cpuid_features(c, false);

[3]
        if (this_cpu->c_bsp_init)
                this_cpu->c_bsp_init(c);

[4]
        setup_force_cpu_cap(X86_FEATURE_ALWAYS);
[5]
}

and we know it from your testing that moving [0] to [5] fixes the crash.

The question is, can we move it to [4], [3], [2] or even [1] instead, without 
breaking the system?

I still don't see where the breakage comes from, but this would help us narrow it 
down.

Thanks,

	Ingo
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