Re: 2.6.14: CR4 not needed to be inspected on the 486 anymore?

[Zachary Amsden <zach@vmware.com>]

Maciej W. Rozycki wrote:

> On Thu, 3 Nov 2005, Zachary Amsden wrote:
>
>  
>
> > > disables code to retrieve the actual value of CR4 on 486-class systems
> > > (which may or may not implement the register, depending on the exact CPU
> > > type and stepping).  This seems suspicious to me, but I have to admit I
> > > haven't followed the discussion on the issue if there was any.
> > >
> > >
> > >      
> > This was deliberate.  CR4 doesn't exist on standard 486 class systems, 
> > and I'm not sure how you could make use of it anyway, since the features 
> > used by Linux - machine check, page size extensions, time stamp counter, 
> > global pages, are only available in Pentium and later class systems, and 
> > identified by CPUID, which also doesn't exist on 486.
> >    
>
> Later Intel i486DX2 and i486DX4 processors (the so called "write-back
> enhanced" ones) did support page size extensions (4MB pages) and as far as
> I know we do use them on such chips.  They did implement the CPUID
> instruction, too (as well as late i486SX and i486SX2 chips that did not
> support PSE).  That's a counter-example that proves the actual value of
> CR4 is going to be useful on these systems.  These chips also implemented
> the VME feature (the PVI and VME bits of CR4), but they may not be
> terribly useful for us.
>
> I used quite a few of such chips myself in mid 90s -- you may search
> archives of various mailing lists for examples of "cpuinfo" dumps for such
> processors.

Finally got the relevant information from Intel. The embedded i486DX-2/4 
series does support CPUID, and PSE, and thus CR4. Ugh. Also just found 
this guy:

$ cat /proc/cpuinfo 
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 4
model           : 7
model name      : 486 DX/2-WB
stepping        : 0
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 1
wp              : yes
flags           : fpu vme pse
bogomips        : 32.25


He won't like me too much. I'll write up a proper patch for this, a la 
mode de rdmsr_safe / wrmsr_safe. At least it's not a bug, just missing 
information.

Zach
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