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

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Date:	Wed, 30 Jan 2008 05:56:57 +0800
From:	Yi Yang <yi.y.yang@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	tglx@...utronix.de, mingo@...hat.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 2.6.24] x86: add sysfs interface for cpuid module

On Tue, 2008-01-29 at 07:51 -0800, H. Peter Anvin wrote:
> Yi Yang wrote:
> > Current cpuid module will create a char device for every logical cpu,
> > when a user cats /dev/cpu/*/cpuid, he/she will enter a limitless loop,
> > the root cause is that cpuid module doesn't decide wether a cpuid level
> > is valid, it just uses an offset to denote cpuid level and take it to
> > cpuid instruction, cpuid instruction will ignore it and return some data
> > 
> > This patch uses sysfs to avoid limitless loop and provide more flexible
> > interface for cpuid, please consider to merge to -mm tree in order to test.
> 
> This is broken.
> 
> Triple broken.
> 
> 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.

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