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    Subject[PATCH 1/2] boot: ignore early NMIs

    Subject: [PATCH] boot: ignore early NMIs

    From: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>

    NMIs very early in the boot process are rarely critical (usually
    it just means that there was a spurious bit flip somewhere in the
    hardware, or that this is a kdump kernel and we received an NMI
    generated in the previous context), so the current behavior of
    halting the system when one occurs is probably a bit over the top.

    This patch changes the early IDT so that NMIs are ignored and the
    kernel can, hopefully, continue executing other code. Harsher
    measures (panic, etc) are defered to the final NMI handler, which
    can actually make an informed decision.

    This issue presented itself in our environment as seemingly
    random hangs in kdump.

    Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
    ---

    diff -urNp linux-3.3-rc6-orig/arch/x86/kernel/head64.c linux-3.3-rc6/arch/x86/kernel/head64.c
    --- linux-3.3-rc6-orig/arch/x86/kernel/head64.c 2012-03-07 15:49:01.834241787 +0900
    +++ linux-3.3-rc6/arch/x86/kernel/head64.c 2012-03-07 18:39:03.173732875 +0900
    @@ -71,7 +71,7 @@ void __init x86_64_start_kernel(char * r
    (__START_KERNEL & PGDIR_MASK)));
    BUILD_BUG_ON(__fix_to_virt(__end_of_fixed_addresses) <= MODULES_END);

    - /* clear bss before set_intr_gate with early_idt_handler */
    + /* clear bss before set_intr_gate with early_idt_handlers */
    clear_bss();

    /* Make NULL pointers segfault */
    @@ -79,13 +79,8 @@ void __init x86_64_start_kernel(char * r

    max_pfn_mapped = KERNEL_IMAGE_SIZE >> PAGE_SHIFT;

    - for (i = 0; i < NUM_EXCEPTION_VECTORS; i++) {
    -#ifdef CONFIG_EARLY_PRINTK
    + for (i = 0; i < NUM_EXCEPTION_VECTORS; i++)
    set_intr_gate(i, &early_idt_handlers[i]);
    -#else
    - set_intr_gate(i, early_idt_handler);
    -#endif
    - }
    load_idt((const struct desc_ptr *)&idt_descr);

    if (console_loglevel == 10)
    diff -urNp linux-3.3-rc6-orig/arch/x86/kernel/head_64.S linux-3.3-rc6/arch/x86/kernel/head_64.S
    --- linux-3.3-rc6-orig/arch/x86/kernel/head_64.S 2012-03-07 15:49:01.838241839 +0900
    +++ linux-3.3-rc6/arch/x86/kernel/head_64.S 2012-03-07 18:41:21.811516876 +0900
    @@ -270,18 +270,29 @@ bad_address:
    jmp bad_address

    .section ".init.text","ax"
    -#ifdef CONFIG_EARLY_PRINTK
    .globl early_idt_handlers
    early_idt_handlers:
    - i = 0
    + vector = 0
    .rept NUM_EXCEPTION_VECTORS
    - movl $i, %esi
    - jmp early_idt_handler
    - i = i + 1
    + /*
    + * NMIs (vector 2) this early in the boot process are rarely critical
    + * (usually it just means that there was a spurious bit flip somewhere
    + * in the hardware, or that this is a kdump kernel and we received an
    + * NMI generated in the previous context), so we ignore them here and
    + * try to continue (see early_nmi_handler implementation below).
    + * Harsher measures (panic, etc) are defered to the final NMI handler,
    + * which can actually make an informed decision.
    + */
    + .if vector == 2
    + jmp early_nmi_handler
    + .else
    + movl $vector, %esi
    + jmp early_exception_handler
    + .endif
    + vector = vector + 1
    .endr
    -#endif

    -ENTRY(early_idt_handler)
    +early_exception_handler:
    #ifdef CONFIG_EARLY_PRINTK
    cmpl $2,early_recursion_flag(%rip)
    jz 1f
    @@ -315,6 +326,9 @@ ENTRY(early_idt_handler)
    1: hlt
    jmp 1b

    +early_nmi_handler:
    + iretq
    +
    #ifdef CONFIG_EARLY_PRINTK
    early_recursion_flag:
    .long 0
    \
     
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      Last update: 2012-03-07 11:57    [W:0.046 / U:62.764 seconds]
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