Messages in this thread | | | Date | Sun, 24 Feb 2008 16:57:40 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [BUG] 2.6.25-rc2-git8 fails to boot on 486 due to TSC breakage |
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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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