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

[Ingo Molnar]

* Borislav Petkov <bp@xxxxxxxxx> wrote:

> On Mon, Jun 29, 2015 at 10:25:29AM +0200, Mike Galbraith wrote:
> > On Mon, 2015-06-29 at 08:40 +0200, Ingo Molnar wrote:
> > > *
> > > 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);
> 
> > [0] is the only spot that breaks box.
> 
> I bet it is that
> 
>         /* Unmask CPUID levels if masked: */
>         if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
>                 if (msr_clear_bit(MSR_IA32_MISC_ENABLE,
>                                   MSR_IA32_MISC_ENABLE_LIMIT_CPUID_BIT) > 0) {
>                         c->cpuid_level = cpuid_eax(0);
>                         get_cpu_cap(c);
>                 }
>         }
> 
> in early_init_intel(). If you feel like playing, you might comment it
> out to see what happens.
> 
> :-)

Indeed, I bet that makes a difference!

I wish that 'unmasking' logic came with more comments:

  - Why do BIOSen ever mask CPUIDs?

  - Why do we unmask the masking?

  - Why doesn't the kernel keep on working just fine even if certain CPUID aspects 
    are turned off?

Thanks,

	Ingo
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