Messages in this thread | | | Date | Mon, 07 Nov 2005 09:32:30 -0800 | From | Zachary Amsden <> | Subject | Re: 2.6.14: CR4 not needed to be inspected on the 486 anymore? |
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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?
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