Re: [PATCH 2/2] cpu: intel, amd: mask cleared cpuid features

From: H. Peter Anvin
Date: Fri Jul 20 2012 - 16:19:34 EST

On 07/20/2012 11:21 AM, Vladimir Davydov wrote:

I am a bit concerned about this patch:

1. it silently changes existing behavior.

Yes, but who needs the current implementation of 'clearcpuid' which,
in fact, just hides flags in /proc/cpuinfo while userspace apps will
see and consequently use all CPU features?

Anyone who wants to disable a feature from the kernel, specifically.

So, I think it logically extends the existing behavior.

2. even on enabled hardware, only some of the bits are maskable.

The patch makes only words 0, 1, 4, 6 maskable, but words 3, 7, 8 are Linux-defined, words 2 and 5 are Transmeta-, Centaur-, etc- defined, and word 9 contains some bizarre Intel CPU features. Thus, it is words 0, 1, 4, 6 that contain useful information for most hardware models.

"Bizarre"? New features, perhaps.

If you ask about some Intel CPUs that can't mask CPUID function 0x80000001, this function describes AMD-specific features, and I bet those Intel CPUs just don't have them at all and thus have nothing to mask.

Not quite.


H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.

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