• Mikael Pettersson's avatar
    x86: fix boot failure on 486 due to TSC breakage · 12c247a6
    Mikael Pettersson authored
     > Diffing dmesg between git7 and git8 doesn't sched any light since
     > git8 also removed the printouts of the x86 caps as they were being
     > initialised and updated. I'm currently adding those printouts back
     > in the hope of seeing where and when the caps get broken.
    
    That turned out to be very illuminating:
    
     --- dmesg-2.6.24-git7	2008-02-24 18:01:25.295851000 +0100
     +++ dmesg-2.6.24-git8	2008-02-24 18:01:25.530358000 +0100
     ...
     CPU: After generic identify, caps: 00000003 00000000 00000000 00000000 00000000 00000000 00000000 00000000
    
     CPU: After all inits, caps: 00000003 00000000 00000000 00000000 00000000 00000000 00000000 00000000
    +CPU: After applying cleared_cpu_caps, caps: 00000013 00000000 00000000 00000000 00000000 00000000 00000000 00000000
    
    Notice how the TSC cap bit goes from Off to On.
    
    (The first two lines are printout loops from -git7 forward-ported
    to -git8, the third line is the same printout loop added just after
    the xor-with-cleared_cpu_caps[] loop.)
    
    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.
    
    [ mingo@elte.hu: fixed a similar bug in setup_64.c as well. ]
    
    The breakage was introduced via commit 7d851c8d.
    Signed-off-by: default avatarMikael Pettersson <mikpe@it.uu.se>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    12c247a6
setup_64.c 28.9 KB