arch/x86/kernel/nmi.c - maze/linux - Git at Google

 /*
  *  Copyright (C) 1991, 1992  Linus Torvalds
  *  Copyright (C) 2000, 2001, 2002 Andi Kleen, SuSE Labs
  *  Copyright (C) 2011	Don Zickus Red Hat, Inc.
  *
  *  Pentium III FXSR, SSE support
  *	Gareth Hughes <gareth@valinux.com>, May 2000
  */

 /*
  * Handle hardware traps and faults.
  */
 #include <linux/spinlock.h>
 #include <linux/kprobes.h>
 #include <linux/kdebug.h>
 #include <linux/nmi.h>
 #include <linux/delay.h>
 #include <linux/hardirq.h>
 #include <linux/slab.h>
 #include <linux/export.h>

 #include <linux/mca.h>

 #if defined(CONFIG_EDAC)
 #include <linux/edac.h>
 #endif

 #include <linux/atomic.h>
 #include <asm/traps.h>
 #include <asm/mach_traps.h>
 #include <asm/nmi.h>

 #define NMI_MAX_NAMELEN	16
 struct nmiaction {
 	struct list_head list;
 	nmi_handler_t handler;
 	unsigned int flags;
 	char *name;
 };

 struct nmi_desc {
 	spinlock_t lock;
 	struct list_head head;
 };

 static struct nmi_desc nmi_desc[NMI_MAX] =
 {
 	{
 		.lock = __SPIN_LOCK_UNLOCKED(&nmi_desc[0].lock),
 		.head = LIST_HEAD_INIT(nmi_desc[0].head),
 	},
 	{
 		.lock = __SPIN_LOCK_UNLOCKED(&nmi_desc[1].lock),
 		.head = LIST_HEAD_INIT(nmi_desc[1].head),
 	},

 };

 struct nmi_stats {
 	unsigned int normal;
 	unsigned int unknown;
 	unsigned int external;
 	unsigned int swallow;
 };

 static DEFINE_PER_CPU(struct nmi_stats, nmi_stats);

 static int ignore_nmis;

 int unknown_nmi_panic;
 /*
  * Prevent NMI reason port (0x61) being accessed simultaneously, can
  * only be used in NMI handler.
  */
 static DEFINE_RAW_SPINLOCK(nmi_reason_lock);

 static int __init setup_unknown_nmi_panic(char *str)
 {
 	unknown_nmi_panic = 1;
 	return 1;
 }
 __setup("unknown_nmi_panic", setup_unknown_nmi_panic);

 #define nmi_to_desc(type) (&nmi_desc[type])

 static int notrace __kprobes nmi_handle(unsigned int type, struct pt_regs *regs, bool b2b)
 {
 	struct nmi_desc *desc = nmi_to_desc(type);
 	struct nmiaction *a;
 	int handled=0;

 	rcu_read_lock();

 	/*
 	 * NMIs are edge-triggered, which means if you have enough
 	 * of them concurrently, you can lose some because only one
 	 * can be latched at any given time.  Walk the whole list
 	 * to handle those situations.
 	 */
 	list_for_each_entry_rcu(a, &desc->head, list)
 		handled += a->handler(type, regs);

 	rcu_read_unlock();

 	/* return total number of NMI events handled */
 	return handled;
 }

 static int __setup_nmi(unsigned int type, struct nmiaction *action)
 {
 	struct nmi_desc *desc = nmi_to_desc(type);
 	unsigned long flags;

 	spin_lock_irqsave(&desc->lock, flags);

 	/*
 	 * most handlers of type NMI_UNKNOWN never return because
 	 * they just assume the NMI is theirs.  Just a sanity check
 	 * to manage expectations
 	 */
 	WARN_ON_ONCE(type == NMI_UNKNOWN && !list_empty(&desc->head));

 	/*
 	 * some handlers need to be executed first otherwise a fake
 	 * event confuses some handlers (kdump uses this flag)
 	 */
 	if (action->flags & NMI_FLAG_FIRST)
 		list_add_rcu(&action->list, &desc->head);
 	else
 		list_add_tail_rcu(&action->list, &desc->head);

 	spin_unlock_irqrestore(&desc->lock, flags);
 	return 0;
 }

 static struct nmiaction *__free_nmi(unsigned int type, const char *name)
 {
 	struct nmi_desc *desc = nmi_to_desc(type);
 	struct nmiaction *n;
 	unsigned long flags;

 	spin_lock_irqsave(&desc->lock, flags);

 	list_for_each_entry_rcu(n, &desc->head, list) {
 		/*
 		 * the name passed in to describe the nmi handler
 		 * is used as the lookup key
 		 */
 		if (!strcmp(n->name, name)) {
 			WARN(in_nmi(),
 				"Trying to free NMI (%s) from NMI context!\n", n->name);
 			list_del_rcu(&n->list);
 			break;
 		}
 	}

 	spin_unlock_irqrestore(&desc->lock, flags);
 	synchronize_rcu();
 	return (n);
 }

 int register_nmi_handler(unsigned int type, nmi_handler_t handler,
 			unsigned long nmiflags, const char *devname)
 {
 	struct nmiaction *action;
 	int retval = -ENOMEM;

 	if (!handler)
 		return -EINVAL;

 	action = kzalloc(sizeof(struct nmiaction), GFP_KERNEL);
 	if (!action)
 		goto fail_action;

 	action->handler = handler;
 	action->flags = nmiflags;
 	action->name = kstrndup(devname, NMI_MAX_NAMELEN, GFP_KERNEL);
 	if (!action->name)
 		goto fail_action_name;

 	retval = __setup_nmi(type, action);

 	if (retval)
 		goto fail_setup_nmi;

 	return retval;

 fail_setup_nmi:
 	kfree(action->name);
 fail_action_name:
 	kfree(action);
 fail_action:

 	return retval;
 }
 EXPORT_SYMBOL_GPL(register_nmi_handler);

 void unregister_nmi_handler(unsigned int type, const char *name)
 {
 	struct nmiaction *a;

 	a = __free_nmi(type, name);
 	if (a) {
 		kfree(a->name);
 		kfree(a);
 	}
 }

 EXPORT_SYMBOL_GPL(unregister_nmi_handler);

 static notrace __kprobes void
 pci_serr_error(unsigned char reason, struct pt_regs *regs)
 {
 	pr_emerg("NMI: PCI system error (SERR) for reason %02x on CPU %d.\n",
 		 reason, smp_processor_id());

 	/*
 	 * On some machines, PCI SERR line is used to report memory
 	 * errors. EDAC makes use of it.
 	 */
 #if defined(CONFIG_EDAC)
 	if (edac_handler_set()) {
 		edac_atomic_assert_error();
 		return;
 	}
 #endif

 	if (panic_on_unrecovered_nmi)
 		panic("NMI: Not continuing");

 	pr_emerg("Dazed and confused, but trying to continue\n");

 	/* Clear and disable the PCI SERR error line. */
 	reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_SERR;
 	outb(reason, NMI_REASON_PORT);
 }

 static notrace __kprobes void
 io_check_error(unsigned char reason, struct pt_regs *regs)
 {
 	unsigned long i;

 	pr_emerg(
 	"NMI: IOCK error (debug interrupt?) for reason %02x on CPU %d.\n",
 		 reason, smp_processor_id());
 	show_registers(regs);

 	if (panic_on_io_nmi)
 		panic("NMI IOCK error: Not continuing");

 	/* Re-enable the IOCK line, wait for a few seconds */
 	reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_IOCHK;
 	outb(reason, NMI_REASON_PORT);

 	i = 20000;
 	while (--i) {
 		touch_nmi_watchdog();
 		udelay(100);
 	}

 	reason &= ~NMI_REASON_CLEAR_IOCHK;
 	outb(reason, NMI_REASON_PORT);
 }

 static notrace __kprobes void
 unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
 {
 	int handled;

 	/*
 	 * Use 'false' as back-to-back NMIs are dealt with one level up.
 	 * Of course this makes having multiple 'unknown' handlers useless
 	 * as only the first one is ever run (unless it can actually determine
 	 * if it caused the NMI)
 	 */
 	handled = nmi_handle(NMI_UNKNOWN, regs, false);
 	if (handled) {
 		__this_cpu_add(nmi_stats.unknown, handled);
 		return;
 	}

 	__this_cpu_add(nmi_stats.unknown, 1);

 #ifdef CONFIG_MCA
 	/*
 	 * Might actually be able to figure out what the guilty party
 	 * is:
 	 */
 	if (MCA_bus) {
 		mca_handle_nmi();
 		return;
 	}
 #endif
 	pr_emerg("Uhhuh. NMI received for unknown reason %02x on CPU %d.\n",
 		 reason, smp_processor_id());

 	pr_emerg("Do you have a strange power saving mode enabled?\n");
 	if (unknown_nmi_panic || panic_on_unrecovered_nmi)
 		panic("NMI: Not continuing");

 	pr_emerg("Dazed and confused, but trying to continue\n");
 }

 static DEFINE_PER_CPU(bool, swallow_nmi);
 static DEFINE_PER_CPU(unsigned long, last_nmi_rip);

 static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
 {
 	unsigned char reason = 0;
 	int handled;
 	bool b2b = false;

 	/*
 	 * CPU-specific NMI must be processed before non-CPU-specific
 	 * NMI, otherwise we may lose it, because the CPU-specific
 	 * NMI can not be detected/processed on other CPUs.
 	 */

 	/*
 	 * Back-to-back NMIs are interesting because they can either
 	 * be two NMI or more than two NMIs (any thing over two is dropped
 	 * due to NMI being edge-triggered).  If this is the second half
 	 * of the back-to-back NMI, assume we dropped things and process
 	 * more handlers.  Otherwise reset the 'swallow' NMI behaviour
 	 */
 	if (regs->ip == __this_cpu_read(last_nmi_rip))
 		b2b = true;
 	else
 		__this_cpu_write(swallow_nmi, false);

 	__this_cpu_write(last_nmi_rip, regs->ip);

 	handled = nmi_handle(NMI_LOCAL, regs, b2b);
 	__this_cpu_add(nmi_stats.normal, handled);
 	if (handled) {
 		/*
 		 * There are cases when a NMI handler handles multiple
 		 * events in the current NMI.  One of these events may
 		 * be queued for in the next NMI.  Because the event is
 		 * already handled, the next NMI will result in an unknown
 		 * NMI.  Instead lets flag this for a potential NMI to
 		 * swallow.
 		 */
 		if (handled > 1)
 			__this_cpu_write(swallow_nmi, true);
 		return;
 	}

 	/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
 	raw_spin_lock(&nmi_reason_lock);
 	reason = get_nmi_reason();

 	if (reason & NMI_REASON_MASK) {
 		if (reason & NMI_REASON_SERR)
 			pci_serr_error(reason, regs);
 		else if (reason & NMI_REASON_IOCHK)
 			io_check_error(reason, regs);
 #ifdef CONFIG_X86_32
 		/*
 		 * Reassert NMI in case it became active
 		 * meanwhile as it's edge-triggered:
 		 */
 		reassert_nmi();
 #endif
 		__this_cpu_add(nmi_stats.external, 1);
 		raw_spin_unlock(&nmi_reason_lock);
 		return;
 	}
 	raw_spin_unlock(&nmi_reason_lock);

 	/*
 	 * Only one NMI can be latched at a time.  To handle
 	 * this we may process multiple nmi handlers at once to
 	 * cover the case where an NMI is dropped.  The downside
 	 * to this approach is we may process an NMI prematurely,
 	 * while its real NMI is sitting latched.  This will cause
 	 * an unknown NMI on the next run of the NMI processing.
 	 *
 	 * We tried to flag that condition above, by setting the
 	 * swallow_nmi flag when we process more than one event.
 	 * This condition is also only present on the second half
 	 * of a back-to-back NMI, so we flag that condition too.
 	 *
 	 * If both are true, we assume we already processed this
 	 * NMI previously and we swallow it.  Otherwise we reset
 	 * the logic.
 	 *
 	 * There are scenarios where we may accidentally swallow
 	 * a 'real' unknown NMI.  For example, while processing
 	 * a perf NMI another perf NMI comes in along with a
 	 * 'real' unknown NMI.  These two NMIs get combined into
 	 * one (as descibed above).  When the next NMI gets
 	 * processed, it will be flagged by perf as handled, but
 	 * noone will know that there was a 'real' unknown NMI sent
 	 * also.  As a result it gets swallowed.  Or if the first
 	 * perf NMI returns two events handled then the second
 	 * NMI will get eaten by the logic below, again losing a
 	 * 'real' unknown NMI.  But this is the best we can do
 	 * for now.
 	 */
 	if (b2b && __this_cpu_read(swallow_nmi))
 		__this_cpu_add(nmi_stats.swallow, 1);
 	else
 		unknown_nmi_error(reason, regs);
 }

 dotraplinkage notrace __kprobes void
 do_nmi(struct pt_regs *regs, long error_code)
 {
 	nmi_enter();

 	inc_irq_stat(__nmi_count);

 	if (!ignore_nmis)
 		default_do_nmi(regs);

 	nmi_exit();
 }

 void stop_nmi(void)
 {
 	ignore_nmis++;
 }

 void restart_nmi(void)
 {
 	ignore_nmis--;
 }

 /* reset the back-to-back NMI logic */
 void local_touch_nmi(void)
 {
 	__this_cpu_write(last_nmi_rip, 0);
 }
	/*
	* Copyright (C) 1991, 1992 Linus Torvalds
	* Copyright (C) 2000, 2001, 2002 Andi Kleen, SuSE Labs
	* Copyright (C) 2011 Don Zickus Red Hat, Inc.
	*
	* Pentium III FXSR, SSE support
	* Gareth Hughes <gareth@valinux.com>, May 2000
	*/

	/*
	* Handle hardware traps and faults.
	*/
	#include <linux/spinlock.h>
	#include <linux/kprobes.h>
	#include <linux/kdebug.h>
	#include <linux/nmi.h>
	#include <linux/delay.h>
	#include <linux/hardirq.h>
	#include <linux/slab.h>
	#include <linux/export.h>

	#include <linux/mca.h>

	#if defined(CONFIG_EDAC)
	#include <linux/edac.h>
	#endif

	#include <linux/atomic.h>
	#include <asm/traps.h>
	#include <asm/mach_traps.h>
	#include <asm/nmi.h>

	#define NMI_MAX_NAMELEN 16
	struct nmiaction {
	struct list_head list;
	nmi_handler_t handler;
	unsigned int flags;
	char *name;
	};

	struct nmi_desc {
	spinlock_t lock;
	struct list_head head;
	};

	static struct nmi_desc nmi_desc[NMI_MAX] =
	{
	{
	.lock = __SPIN_LOCK_UNLOCKED(&nmi_desc[0].lock),
	.head = LIST_HEAD_INIT(nmi_desc[0].head),
	},
	{
	.lock = __SPIN_LOCK_UNLOCKED(&nmi_desc[1].lock),
	.head = LIST_HEAD_INIT(nmi_desc[1].head),
	},

	};

	struct nmi_stats {
	unsigned int normal;
	unsigned int unknown;
	unsigned int external;
	unsigned int swallow;
	};

	static DEFINE_PER_CPU(struct nmi_stats, nmi_stats);

	static int ignore_nmis;

	int unknown_nmi_panic;
	/*
	* Prevent NMI reason port (0x61) being accessed simultaneously, can
	* only be used in NMI handler.
	*/
	static DEFINE_RAW_SPINLOCK(nmi_reason_lock);

	static int __init setup_unknown_nmi_panic(char *str)
	{
	unknown_nmi_panic = 1;
	return 1;
	}
	__setup("unknown_nmi_panic", setup_unknown_nmi_panic);

	#define nmi_to_desc(type) (&nmi_desc[type])

	static int notrace __kprobes nmi_handle(unsigned int type, struct pt_regs *regs, bool b2b)
	{
	struct nmi_desc *desc = nmi_to_desc(type);
	struct nmiaction *a;
	int handled=0;

	rcu_read_lock();

	/*
	* NMIs are edge-triggered, which means if you have enough
	* of them concurrently, you can lose some because only one
	* can be latched at any given time. Walk the whole list
	* to handle those situations.
	*/
	list_for_each_entry_rcu(a, &desc->head, list)
	handled += a->handler(type, regs);

	rcu_read_unlock();

	/* return total number of NMI events handled */
	return handled;
	}

	static int __setup_nmi(unsigned int type, struct nmiaction *action)
	{
	struct nmi_desc *desc = nmi_to_desc(type);
	unsigned long flags;

	spin_lock_irqsave(&desc->lock, flags);

	/*
	* most handlers of type NMI_UNKNOWN never return because
	* they just assume the NMI is theirs. Just a sanity check
	* to manage expectations
	*/
	WARN_ON_ONCE(type == NMI_UNKNOWN && !list_empty(&desc->head));

	/*
	* some handlers need to be executed first otherwise a fake
	* event confuses some handlers (kdump uses this flag)
	*/
	if (action->flags & NMI_FLAG_FIRST)
	list_add_rcu(&action->list, &desc->head);
	else
	list_add_tail_rcu(&action->list, &desc->head);

	spin_unlock_irqrestore(&desc->lock, flags);
	return 0;
	}

	static struct nmiaction __free_nmi(unsigned int type, const char name)
	{
	struct nmi_desc *desc = nmi_to_desc(type);
	struct nmiaction *n;
	unsigned long flags;

	spin_lock_irqsave(&desc->lock, flags);

	list_for_each_entry_rcu(n, &desc->head, list) {
	/*
	* the name passed in to describe the nmi handler
	* is used as the lookup key
	*/
	if (!strcmp(n->name, name)) {
	WARN(in_nmi(),
	"Trying to free NMI (%s) from NMI context!\n", n->name);
	list_del_rcu(&n->list);
	break;
	}
	}

	spin_unlock_irqrestore(&desc->lock, flags);
	synchronize_rcu();
	return (n);
	}

	int register_nmi_handler(unsigned int type, nmi_handler_t handler,
	unsigned long nmiflags, const char *devname)
	{
	struct nmiaction *action;
	int retval = -ENOMEM;

	if (!handler)
	return -EINVAL;

	action = kzalloc(sizeof(struct nmiaction), GFP_KERNEL);
	if (!action)
	goto fail_action;

	action->handler = handler;
	action->flags = nmiflags;
	action->name = kstrndup(devname, NMI_MAX_NAMELEN, GFP_KERNEL);
	if (!action->name)
	goto fail_action_name;

	retval = __setup_nmi(type, action);

	if (retval)
	goto fail_setup_nmi;

	return retval;

	fail_setup_nmi:
	kfree(action->name);
	fail_action_name:
	kfree(action);
	fail_action:

	return retval;
	}
	EXPORT_SYMBOL_GPL(register_nmi_handler);

	void unregister_nmi_handler(unsigned int type, const char *name)
	{
	struct nmiaction *a;

	a = __free_nmi(type, name);
	if (a) {
	kfree(a->name);
	kfree(a);
	}
	}

	EXPORT_SYMBOL_GPL(unregister_nmi_handler);

	static notrace __kprobes void
	pci_serr_error(unsigned char reason, struct pt_regs *regs)
	{
	pr_emerg("NMI: PCI system error (SERR) for reason %02x on CPU %d.\n",
	reason, smp_processor_id());

	/*
	* On some machines, PCI SERR line is used to report memory
	* errors. EDAC makes use of it.
	*/
	#if defined(CONFIG_EDAC)
	if (edac_handler_set()) {
	edac_atomic_assert_error();
	return;
	}
	#endif

	if (panic_on_unrecovered_nmi)
	panic("NMI: Not continuing");

	pr_emerg("Dazed and confused, but trying to continue\n");

	/* Clear and disable the PCI SERR error line. */
	reason = (reason & NMI_REASON_CLEAR_MASK) \| NMI_REASON_CLEAR_SERR;
	outb(reason, NMI_REASON_PORT);
	}

	static notrace __kprobes void
	io_check_error(unsigned char reason, struct pt_regs *regs)
	{
	unsigned long i;

	pr_emerg(
	"NMI: IOCK error (debug interrupt?) for reason %02x on CPU %d.\n",
	reason, smp_processor_id());
	show_registers(regs);

	if (panic_on_io_nmi)
	panic("NMI IOCK error: Not continuing");

	/* Re-enable the IOCK line, wait for a few seconds */
	reason = (reason & NMI_REASON_CLEAR_MASK) \| NMI_REASON_CLEAR_IOCHK;
	outb(reason, NMI_REASON_PORT);

	i = 20000;
	while (--i) {
	touch_nmi_watchdog();
	udelay(100);
	}

	reason &= ~NMI_REASON_CLEAR_IOCHK;
	outb(reason, NMI_REASON_PORT);
	}

	static notrace __kprobes void
	unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
	{
	int handled;

	/*
	* Use 'false' as back-to-back NMIs are dealt with one level up.
	* Of course this makes having multiple 'unknown' handlers useless
	* as only the first one is ever run (unless it can actually determine
	* if it caused the NMI)
	*/
	handled = nmi_handle(NMI_UNKNOWN, regs, false);
	if (handled) {
	__this_cpu_add(nmi_stats.unknown, handled);
	return;
	}

	__this_cpu_add(nmi_stats.unknown, 1);

	#ifdef CONFIG_MCA
	/*
	* Might actually be able to figure out what the guilty party
	* is:
	*/
	if (MCA_bus) {
	mca_handle_nmi();
	return;
	}
	#endif
	pr_emerg("Uhhuh. NMI received for unknown reason %02x on CPU %d.\n",
	reason, smp_processor_id());

	pr_emerg("Do you have a strange power saving mode enabled?\n");
	if (unknown_nmi_panic \|\| panic_on_unrecovered_nmi)
	panic("NMI: Not continuing");

	pr_emerg("Dazed and confused, but trying to continue\n");
	}

	static DEFINE_PER_CPU(bool, swallow_nmi);
	static DEFINE_PER_CPU(unsigned long, last_nmi_rip);

	static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
	{
	unsigned char reason = 0;
	int handled;
	bool b2b = false;

	/*
	* CPU-specific NMI must be processed before non-CPU-specific
	* NMI, otherwise we may lose it, because the CPU-specific
	* NMI can not be detected/processed on other CPUs.
	*/

	/*
	* Back-to-back NMIs are interesting because they can either
	* be two NMI or more than two NMIs (any thing over two is dropped
	* due to NMI being edge-triggered). If this is the second half
	* of the back-to-back NMI, assume we dropped things and process
	* more handlers. Otherwise reset the 'swallow' NMI behaviour
	*/
	if (regs->ip == __this_cpu_read(last_nmi_rip))
	b2b = true;
	else
	__this_cpu_write(swallow_nmi, false);

	__this_cpu_write(last_nmi_rip, regs->ip);

	handled = nmi_handle(NMI_LOCAL, regs, b2b);
	__this_cpu_add(nmi_stats.normal, handled);
	if (handled) {
	/*
	* There are cases when a NMI handler handles multiple
	* events in the current NMI. One of these events may
	* be queued for in the next NMI. Because the event is
	* already handled, the next NMI will result in an unknown
	* NMI. Instead lets flag this for a potential NMI to
	* swallow.
	*/
	if (handled > 1)
	__this_cpu_write(swallow_nmi, true);
	return;
	}

	/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
	raw_spin_lock(&nmi_reason_lock);
	reason = get_nmi_reason();

	if (reason & NMI_REASON_MASK) {
	if (reason & NMI_REASON_SERR)
	pci_serr_error(reason, regs);
	else if (reason & NMI_REASON_IOCHK)
	io_check_error(reason, regs);
	#ifdef CONFIG_X86_32
	/*
	* Reassert NMI in case it became active
	* meanwhile as it's edge-triggered:
	*/
	reassert_nmi();
	#endif
	__this_cpu_add(nmi_stats.external, 1);
	raw_spin_unlock(&nmi_reason_lock);
	return;
	}
	raw_spin_unlock(&nmi_reason_lock);

	/*
	* Only one NMI can be latched at a time. To handle
	* this we may process multiple nmi handlers at once to
	* cover the case where an NMI is dropped. The downside
	* to this approach is we may process an NMI prematurely,
	* while its real NMI is sitting latched. This will cause
	* an unknown NMI on the next run of the NMI processing.
	*
	* We tried to flag that condition above, by setting the
	* swallow_nmi flag when we process more than one event.
	* This condition is also only present on the second half
	* of a back-to-back NMI, so we flag that condition too.
	*
	* If both are true, we assume we already processed this
	* NMI previously and we swallow it. Otherwise we reset
	* the logic.
	*
	* There are scenarios where we may accidentally swallow
	* a 'real' unknown NMI. For example, while processing
	* a perf NMI another perf NMI comes in along with a
	* 'real' unknown NMI. These two NMIs get combined into
	* one (as descibed above). When the next NMI gets
	* processed, it will be flagged by perf as handled, but
	* noone will know that there was a 'real' unknown NMI sent
	* also. As a result it gets swallowed. Or if the first
	* perf NMI returns two events handled then the second
	* NMI will get eaten by the logic below, again losing a
	* 'real' unknown NMI. But this is the best we can do
	* for now.
	*/
	if (b2b && __this_cpu_read(swallow_nmi))
	__this_cpu_add(nmi_stats.swallow, 1);
	else
	unknown_nmi_error(reason, regs);
	}

	dotraplinkage notrace __kprobes void
	do_nmi(struct pt_regs *regs, long error_code)
	{
	nmi_enter();

	inc_irq_stat(__nmi_count);

	if (!ignore_nmis)
	default_do_nmi(regs);

	nmi_exit();
	}

	void stop_nmi(void)
	{
	ignore_nmis++;
	}

	void restart_nmi(void)
	{
	ignore_nmis--;
	}

	/* reset the back-to-back NMI logic */
	void local_touch_nmi(void)
	{
	__this_cpu_write(last_nmi_rip, 0);
	}