lkml.org 
[lkml]   [2008]   [Feb]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    SubjectRe: [BUG] 2.6.25-rc2-git8 fails to boot on 486 due to TSC breakage

    * Mikael Pettersson <mikpe@it.uu.se> wrote:

    > > yes, i already fixed that when i added Mikael's patch and it's all
    > > queued up.
    >
    > Ok. For reference and for LKML viewers, this is what the final patch
    > should be:

    below is the full commit - will send it to Linus later today.

    Ingo

    --------------->
    Subject: x86: fix boot failure on 486 due to TSC breakage
    From: Mikael Pettersson <mikpe@it.uu.se>
    Date: Sun, 24 Feb 2008 18:27:03 +0100

    > 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 7d851c8d3db0.

    Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    arch/x86/kernel/cpu/common.c | 2 +-
    arch/x86/kernel/setup_64.c | 2 +-
    2 files changed, 2 insertions(+), 2 deletions(-)

    Index: linux-x86.q/arch/x86/kernel/cpu/common.c
    ===================================================================
    --- linux-x86.q.orig/arch/x86/kernel/cpu/common.c
    +++ linux-x86.q/arch/x86/kernel/cpu/common.c
    @@ -504,7 +504,7 @@ void __cpuinit identify_cpu(struct cpuin

    /* Clear all flags overriden by options */
    for (i = 0; i < NCAPINTS; i++)
    - c->x86_capability[i] ^= cleared_cpu_caps[i];
    + c->x86_capability[i] &= ~cleared_cpu_caps[i];

    /* Init Machine Check Exception if available. */
    mcheck_init(c);
    Index: linux-x86.q/arch/x86/kernel/setup_64.c
    ===================================================================
    --- linux-x86.q.orig/arch/x86/kernel/setup_64.c
    +++ linux-x86.q/arch/x86/kernel/setup_64.c
    @@ -1021,7 +1021,7 @@ void __cpuinit identify_cpu(struct cpuin

    /* Clear all flags overriden by options */
    for (i = 0; i < NCAPINTS; i++)
    - c->x86_capability[i] ^= cleared_cpu_caps[i];
    + c->x86_capability[i] &= ~cleared_cpu_caps[i];

    #ifdef CONFIG_X86_MCE
    mcheck_init(c);

    \
     
     \ /
      Last update: 2008-02-25 10:23    [W:0.038 / U:1.348 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site