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    Subjecti387/FPU init issues...
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    Peoplez,

    Ive narrowed down a problem i am having with an old P2 to commit
    61c4628b538608c1a85211ed8438136adfeb9a95 with subject "x86, fpu: split
    FPU state from task struct - v5" (Authored by Suresh and committed by
    Ingo on Apr/19).

    In the process i learnt how painfully time consuming and boring a blind
    git bisect feast could be (the last time a kernel worked on the P2 was
    back in 2.6.23). I literally spent no less than 10 hours tracking this
    (Ok, I was chewing tobbaco in between running git bisect bad/good,
    compile, copy over kernel, spit here, reboot, test).
    Also this patch is so huge that given my lack of knowledge in the area,
    i couldnt do better bisecting to be more exact on what is causing this.
    i.e the patch is not bisect-friendly.
    So the best i can do is have other people take it from here.

    I am able to reproduce the issue consistently on my laptop using qemu
    (which helped speed debugging a bit). I have also narrowed it down to
    include/asm-x86/i387.h::__save_init_fpu in (32 bit version) - it dies
    somewhere in calling the following line:

    ----
    alternative_input(
    "fnsave %[fx] ;fwait;" GENERIC_NOP8 GENERIC_NOP4,
    "fxsave %[fx]\n"
    "bt $7,%[fsw] ; jnc 1f ; fnclex\n1:",
    X86_FEATURE_FXSR,
    [fx] "m" (tsk->thread.xstate->fxsave),
    [fsw] "m" (tsk->thread.xstate->fxsave.swd) : "memory");
    ----------

    The only thing that has changed there compared to good version is the
    last two lines. But that looks sane to me given the struct naming has
    changed. So i am suspecting the calling path perhaps not setting
    something or other.

    ------------ boot output paste ----------------------
    [....]
    Compat vDSO mapped to ffffe000.
    CPU: Intel Pentium II (Klamath) stepping 03
    Checking 'hlt' instruction... OK.
    Freeing SMP alternatives: 0k freed
    invalid opcode: 0000 [#1]
    Modules linked in:

    Pid: 0, comm: swapper Not tainted (2.6.25-00000-g61c4628 #22)
    EIP: 0060:[<c01012d0>] EFLAGS: 00000202 CPU: 0
    EIP is at prepare_to_copy+0x20/0x50
    EAX: c1101880 EBX: fffffff4 ECX: c04eff80 EDX: c04bb3e0
    ESI: c04bb3e0 EDI: c04eff80 EBP: c04efeb0 ESP: c04efeb0
    DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
    Process swapper (pid: 0, ti=c04ee000 task=c04bb3e0 task.ti=c04ee000)
    Stack: c04eff0c c01183a0 00000000 c0543566 00000000 c04eff84 00000296
    c04effa4
    c04eff80 00000000 00800b00 00000001 c04eff5c 00000296 c0543565
    c0543544
    00000026 c04effb4 00000296 c04effd4 00000000 00800b00 c04eff80
    c04eff64
    Call Trace:
    [<c01183a0>] ? copy_process+0x60/0x10d0
    [<c0119504>] ? do_fork+0x54/0x210
    [<c01355cc>] ? lock_release_holdtime+0x6c/0x70
    [<c04f0000>] ? __init_begin+0x0/0x69
    [<c010fe5d>] ? change_page_attr_set_clr+0xcd/0x1e0
    [<c0101996>] ? kernel_thread+0x86/0xa0
    [<c04f0710>] ? kernel_init+0x0/0x270
    [<c04f0710>] ? kernel_init+0x0/0x270
    [<c0103260>] ? kernel_thread_helper+0x0/0x10
    [<c03bb874>] ? rest_init+0x14/0x50
    [<c04f0b7a>] ? start_kernel+0x1fa/0x280
    [<c04f03f0>] ? unknown_bootoption+0x0/0x210
    [<c04f02b8>] ? i386_start_kernel+0x8/0x10
    =======================
    Code: 8d 74 26 00 8d bc 27 00 00 00 00 55 89 c2 8b 40 04 89 e5 f6 40 0c
    01 74 32 8b 82 60 02 00 00 0f ae 00 0f ba 60 02 07 73 02 db e2 <0f> 1f
    00 90 8d b4 26 00 00 00 00 89 f6 8b 42 04 83 60 0c fe 0f
    EIP: [<c01012d0>] prepare_to_copy+0x20/0x50 SS:ESP 0068:c04efeb0
    ---[ end trace ca143223eefdc828 ]---
    Kernel panic - not syncing: Attempted to kill the idle task!
    ------------------------------------------------------------------------

    Remedy:
    I am able to get the system to boot fine if i passed an option to tell
    it there is no i387 and compile in math emulation (but then a few of
    standard programs start segfaulting on me and i dont wanna go chasing
    that).

    Let me know what you want me to try out since i can do this on my laptop
    now.

    cheers,
    jamal



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