Subject: Re: [PATCH 2.6.24] x86: add sysfs interface for cpuid module
From: Yi Yang <yi.y.yang@intel.com>
Reply-To: yi.y.yang@intel.com
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: tglx@linutronix.de, mingo@redhat.com, linux-kernel@vger.kernel.org,
       akpm@linux-foundation.org
In-Reply-To: <47A02483.5000606@zytor.com>
References: <1199441414.19185.9.camel@yangyi-dev.bj.intel.com>
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	 <1201562058.12722.9.camel@yangyi-dev.bj.intel.com>
	 <479F4B99.4060701@zytor.com>
	 <1201643817.12722.30.camel@yangyi-dev.bj.intel.com>
	 <47A02483.5000606@zytor.com>
Content-Type: text/plain
Organization: Intel
Date: Wed, 30 Jan 2008 06:13:13 +0800
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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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