linux-kernel - Re: [BUG] 2.6.25-rc2-git8 fails to boot on 486 due to TSC breakage

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Message-ID: <47C21284.7040607@zytor.com>
Date:	Sun, 24 Feb 2008 16:57:40 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mikael Pettersson <mikpe@...uu.se>
CC:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	mingo@...hat.com, tglx@...utronix.de
Subject: Re: [BUG] 2.6.25-rc2-git8 fails to boot on 486 due to TSC breakage

Mikael Pettersson wrote:
> 
> Here's how the breakage occurs:
> 1. arch/x86/kernel/tsc_32.c:tsc_init() sees !cpu_has_tsc,
>    so bails and calls setup_clear_cpu_cap(X86_FEATURE_TSC).
> 2. include/asm-x86/cpufeature.h:setup_clear_cpu_cap(bit) clears
>    the bit in boot_cpu_data and sets it in cleared_cpu_caps
> 3. arch/x86/kernel/cpu/common.c:identify_cpu() XORs all caps
>    in with cleared_cpu_caps
>    HOWEVER, at this point c->x86_capability correctly has TSC
>    Off, cleared_cpu_caps has TSC On, so the XOR incorrectly
>    sets TSC to On in c->x86_capability, with disastrous results.
> 
> The real bug is that clearing bits with XOR only works if the
> bits are known to be 1 prior to the XOR, and that's not true here.
> 
> A simple fix is to convert the XOR to AND-NOT instead. The following
> patch does that, and allows my 486 to boot 2.6.25-rc kernels again.
> 

Please fix it in both places.  Using XOR instead of AND-NOT is a bug, 
plain and simple.

	-hpa
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