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    SubjectRe: [patch] disable NMI watchdog by default
    Date
    On Monday 05 March 2007 07:20, Ingo Molnar wrote:
    >
    > Linus,
    >
    > Andrew sent the patch below (which is now months old and has been in -mm
    > for some time) towards Andi's tree 4 weeks ago, but apparently it fell
    > into a black hole there - the patch is still not upstream!
    >
    > This is a must-have for v2.6.21

    I agree.
    There are multiple machines not booting because of nmi_watchdog.
    Some of them are documented here:
    http://bugzilla.kernel.org/show_bug.cgi?id=7839

    We used to think this was the "nolapic" bug,
    but it is actually the "nmi_watchdog=0" bug.

    -Len

    > Frankly, i find it ridiculous that i had to write more than 10 emails
    > about this stupid topic already, while i'm the original author of this
    > feature to begin with. The "do not enable by default debug features that
    > break certain systems" concept is obvious to me.
    >
    > This category of regressions has been introduced by Andi when he made
    > the NMI watchdog the default in certain scenarios at around v2.6.18 -
    > over my repeated objections. Andi Cc:-ed :-)
    >
    > Ingo
    >
    > ----- Forwarded message from Ingo Molnar <mingo@elte.hu> -----
    >
    > From: Ingo Molnar <mingo@elte.hu>
    > Subject: [patch] disable NMI watchdog by default
    >
    > there's a new NMI watchdog related problem: KVM crashes on certain
    > bzImages because ... we enable the NMI watchdog by default (even if the
    > user does not ask for it) , and no other OS on this planet does that so
    > KVM doesnt have emulation for that yet. So KVM injects a #GP, which
    > crashes the Linux guest:
    >
    > general protection fault: 0000 [#1]
    > PREEMPT SMP
    > Modules linked in:
    > CPU: 0
    > EIP: 0060:[<c011a8ae>] Not tainted VLI
    > EFLAGS: 00000246 (2.6.20-rc5-rt0 #3)
    > EIP is at setup_apic_nmi_watchdog+0x26d/0x3d3
    >
    > and no, i did /not/ request an nmi_watchdog on the boot command line!
    >
    > Solution: turn off that darn thing! It's a debug tool, not a 'make life
    > harder' tool!!
    >
    > with this patch the KVM guest boots up just fine.
    >
    > And with this my laptop (Lenovo T60) also stopped its sporadic hard
    > hanging (sometimes in acpi_init(), sometimes later during bootup,
    > sometimes much later during actual use) as well. It hung with both
    > nmi_watchdog=1 and nmi_watchdog=2, so it's generally the fact of NMI
    > injection that is causing problems, not the NMI watchdog variant, nor
    > any particular bootup code.
    >
    > The patch is unintrusive.
    >
    > Signed-off-by: Ingo Molnar <mingo@elte.hu>
    >
    > Index: linux/include/asm-i386/nmi.h
    > ===================================================================
    > --- linux.orig/include/asm-i386/nmi.h
    > +++ linux/include/asm-i386/nmi.h
    > @@ -33,7 +33,7 @@ extern int nmi_watchdog_tick (struct pt_
    >
    > extern atomic_t nmi_active;
    > extern unsigned int nmi_watchdog;
    > -#define NMI_DEFAULT -1
    > +#define NMI_DEFAULT 0
    > #define NMI_NONE 0
    > #define NMI_IO_APIC 1
    > #define NMI_LOCAL_APIC 2
    > Index: linux/include/asm-x86_64/nmi.h
    > ===================================================================
    > --- linux.orig/include/asm-x86_64/nmi.h
    > +++ linux/include/asm-x86_64/nmi.h
    > @@ -63,7 +63,7 @@ extern int setup_nmi_watchdog(char *);
    >
    > extern atomic_t nmi_active;
    > extern unsigned int nmi_watchdog;
    > -#define NMI_DEFAULT -1
    > +#define NMI_DEFAULT 0
    > #define NMI_NONE 0
    > #define NMI_IO_APIC 1
    > #define NMI_LOCAL_APIC 2
    >
    > -
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