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

[linux-os (Dick Johnson)]

On Mon, 7 Nov 2005, Zachary Amsden wrote:

> 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.
>

Yes it is. The i486 Programmer's reference manual documents only
to CR3.

> What is immoral about opcode trapping?
>
> Zach
>

Unfortunately Intel and others call bad opcodes "illegal opcodes".
They are not, because as powerful as they are, Intel doesn't make
law. Instead, they are "invalid opcodes" (or just immoral) as I
stated as a play on words.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.55 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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