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

[Zachary Amsden <zach@vmware.com>]

linux-os (Dick Johnson) wrote:

>On Mon, 7 Nov 2005, Zachary Amsden wrote:
>
>  
>
>>Maciej W. Rozycki wrote:
>>
>>    
>>
>>>On Mon, 7 Nov 2005, Zachary Amsden wrote:
>>>
>>>
>>>
>>>      
>>>
>>>>While this is at least no worse in the nested fault case than earlier
>>>>kernels, I really wish I had one of those weird 486s so I could test the
>>>>faulting mechanism.  It seems the trap handling code has gotten quite
>>>>
>>>>
>>>>        
>>>>
>>>What's so weird about 486s?  Besides, for testing it doesn't have to be
>>>one -- you will get away with a 386, too.  I have neither anymore, but
>>>there are people around still using them.
>>>
>>>
>>>      
>>>
>>Because I hold in my hand "i486 Microprocessor Programmer's Reference
>>Manual, c 1990", and it has no mention whatsoever of CR4, and all
>>documentation I had until Friday had either no mention of CR4, or
>>something to the effect of "new on Pentium, the CR4 register ..."  So
>>I've had to re-adjust my definition of 486, which was weird.
>>
>>Zach
>>-
>>    
>>
>
>Yes, and undocumented opcodes might not fault. They might do nothing
>or something strange. It's not a good idea to use an undocumented
>opcode in kernel space. The read-from-CR4 in kernel space, hoping
>that an immoral-opcode trap will save you is not good practice.
>
>You might reset the processor.
>  
>

No, you won't.  #UD and #GP will not (I hesitate to say never, but other 
than a processor bug, I believe that is correct) reset the processor.  
And CR4 is not "undocumented", even on 486.

What is immoral about opcode trapping?

Zach