Messages in this thread | | | Date | Thu, 03 Nov 2005 15:49:17 -0800 | From | Zachary Amsden <> | Subject | Re: 2.6.14: CR4 not needed to be inspected on the 486 anymore? |
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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.
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