Re: [PATCH 2.6.24] x86: add sysfs interface for cpuid module

[Yi Yang]

On Tue, 2008-01-29 at 23:17 -0800, H. Peter Anvin wrote:
> Yi Yang wrote:
> >>
> >> It's broken, because it doesn't take into account the fact that Intel 
> >> broke CPUID level 4 and made it "repeating" (neither did the cpuid char 
> >> device, because it predated the Intel braindamage; I've had a patch for 
> >> it privately for a while, but didn't push it upstream because paravirt 
> >> broke it royally and I wanted the situation to settle down.)
> 
> > level 4 doesn't result in repeating on Intel CPU, cpuid module sets
> > file offset to level, so cat /dev/cpu/*/cpuid will run cpuid instruction
> > continuously.
> 
> The issue is that Intel suddenly made CPUID ECX-sensitive, which there 
> is no way to represent.
Function cpuid has reset ecx to 0 immediate before calling to __cpuid,
so this shouldn't be a problem now.

in include/asm-x86/processor_32.h
/*
 * Generic CPUID function
 * clear %ecx since some cpus (Cyrix MII) do not set or clear %ecx
 * resulting in stale register contents being returned.
 */
static inline void cpuid(unsigned int op,
                         unsigned int *eax, unsigned int *ebx,
                         unsigned int *ecx, unsigned int *edx)
{
        *eax = op;
        *ecx = 0;
        __cpuid(eax, ebx, ecx, edx);
}
> 
> As far as cat /dev/cpu/*/cpuid, that's a user error.
> 
> 	-hpa
> 

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