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

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.

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

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