arch/x86/mm/kmmio.c - kernel/msm-4.9 - Gitiles

 /* Support for MMIO probes.
  * Benfit many code from kprobes
  * (C) 2002 Louis Zhuang <louis.zhuang@intel.com>.
  *     2007 Alexander Eichner
  *     2008 Pekka Paalanen <pq@iki.fi>
  */

 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

 #include <linux/list.h>
 #include <linux/rculist.h>
 #include <linux/spinlock.h>
 #include <linux/hash.h>
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/kernel.h>
 #include <linux/uaccess.h>
 #include <linux/ptrace.h>
 #include <linux/preempt.h>
 #include <linux/percpu.h>
 #include <linux/kdebug.h>
 #include <linux/mutex.h>
 #include <linux/io.h>
 #include <asm/cacheflush.h>
 #include <asm/tlbflush.h>
 #include <linux/errno.h>
 #include <asm/debugreg.h>
 #include <linux/mmiotrace.h>

 #define KMMIO_PAGE_HASH_BITS 4
 #define KMMIO_PAGE_TABLE_SIZE (1 << KMMIO_PAGE_HASH_BITS)

 struct kmmio_fault_page {
 	struct list_head list;
 	struct kmmio_fault_page *release_next;
 	unsigned long page; /* location of the fault page */
 	pteval_t old_presence; /* page presence prior to arming */
 	bool armed;

 	/*
 	 * Number of times this page has been registered as a part
 	 * of a probe. If zero, page is disarmed and this may be freed.
 	 * Used only by writers (RCU) and post_kmmio_handler().
 	 * Protected by kmmio_lock, when linked into kmmio_page_table.
 	 */
 	int count;
 };

 struct kmmio_delayed_release {
 	struct rcu_head rcu;
 	struct kmmio_fault_page *release_list;
 };

 struct kmmio_context {
 	struct kmmio_fault_page *fpage;
 	struct kmmio_probe *probe;
 	unsigned long saved_flags;
 	unsigned long addr;
 	int active;
 };

 static DEFINE_SPINLOCK(kmmio_lock);

 /* Protected by kmmio_lock */
 unsigned int kmmio_count;

 /* Read-protected by RCU, write-protected by kmmio_lock. */
 static struct list_head kmmio_page_table[KMMIO_PAGE_TABLE_SIZE];
 static LIST_HEAD(kmmio_probes);

 static struct list_head *kmmio_page_list(unsigned long page)
 {
 	return &kmmio_page_table[hash_long(page, KMMIO_PAGE_HASH_BITS)];
 }

 /* Accessed per-cpu */
 static DEFINE_PER_CPU(struct kmmio_context, kmmio_ctx);

 /*
  * this is basically a dynamic stabbing problem:
  * Could use the existing prio tree code or
  * Possible better implementations:
  * The Interval Skip List: A Data Structure for Finding All Intervals That
  * Overlap a Point (might be simple)
  * Space Efficient Dynamic Stabbing with Fast Queries - Mikkel Thorup
  */
 /* Get the kmmio at this addr (if any). You must be holding RCU read lock. */
 static struct kmmio_probe *get_kmmio_probe(unsigned long addr)
 {
 	struct kmmio_probe *p;
 	list_for_each_entry_rcu(p, &kmmio_probes, list) {
 		if (addr >= p->addr && addr < (p->addr + p->len))
 			return p;
 	}
 	return NULL;
 }

 /* You must be holding RCU read lock. */
 static struct kmmio_fault_page *get_kmmio_fault_page(unsigned long page)
 {
 	struct list_head *head;
 	struct kmmio_fault_page *f;

 	page &= PAGE_MASK;
 	head = kmmio_page_list(page);
 	list_for_each_entry_rcu(f, head, list) {
 		if (f->page == page)
 			return f;
 	}
 	return NULL;
 }

 static void clear_pmd_presence(pmd_t *pmd, bool clear, pmdval_t *old)
 {
 	pmdval_t v = pmd_val(*pmd);
 	if (clear) {
 		*old = v & _PAGE_PRESENT;
 		v &= ~_PAGE_PRESENT;
 	} else	/* presume this has been called with clear==true previously */
 		v |= *old;
 	set_pmd(pmd, __pmd(v));
 }

 static void clear_pte_presence(pte_t *pte, bool clear, pteval_t *old)
 {
 	pteval_t v = pte_val(*pte);
 	if (clear) {
 		*old = v & _PAGE_PRESENT;
 		v &= ~_PAGE_PRESENT;
 	} else	/* presume this has been called with clear==true previously */
 		v |= *old;
 	set_pte_atomic(pte, __pte(v));
 }

 static int clear_page_presence(struct kmmio_fault_page *f, bool clear)
 {
 	unsigned int level;
 	pte_t *pte = lookup_address(f->page, &level);

 	if (!pte) {
 		pr_err("no pte for page 0x%08lx\n", f->page);
 		return -1;
 	}

 	switch (level) {
 	case PG_LEVEL_2M:
 		clear_pmd_presence((pmd_t *)pte, clear, &f->old_presence);
 		break;
 	case PG_LEVEL_4K:
 		clear_pte_presence(pte, clear, &f->old_presence);
 		break;
 	default:
 		pr_err("unexpected page level 0x%x.\n", level);
 		return -1;
 	}

 	__flush_tlb_one(f->page);
 	return 0;
 }

 /*
  * Mark the given page as not present. Access to it will trigger a fault.
  *
  * Struct kmmio_fault_page is protected by RCU and kmmio_lock, but the
  * protection is ignored here. RCU read lock is assumed held, so the struct
  * will not disappear unexpectedly. Furthermore, the caller must guarantee,
  * that double arming the same virtual address (page) cannot occur.
  *
  * Double disarming on the other hand is allowed, and may occur when a fault
  * and mmiotrace shutdown happen simultaneously.
  */
 static int arm_kmmio_fault_page(struct kmmio_fault_page *f)
 {
 	int ret;
 	WARN_ONCE(f->armed, KERN_ERR pr_fmt("kmmio page already armed.\n"));
 	if (f->armed) {
 		pr_warning("double-arm: page 0x%08lx, ref %d, old %d\n",
 			   f->page, f->count, !!f->old_presence);
 	}
 	ret = clear_page_presence(f, true);
 	WARN_ONCE(ret < 0, KERN_ERR pr_fmt("arming 0x%08lx failed.\n"),
 		  f->page);
 	f->armed = true;
 	return ret;
 }

 /** Restore the given page to saved presence state. */
 static void disarm_kmmio_fault_page(struct kmmio_fault_page *f)
 {
 	int ret = clear_page_presence(f, false);
 	WARN_ONCE(ret < 0,
 			KERN_ERR "kmmio disarming 0x%08lx failed.\n", f->page);
 	f->armed = false;
 }

 /*
  * This is being called from do_page_fault().
  *
  * We may be in an interrupt or a critical section. Also prefecthing may
  * trigger a page fault. We may be in the middle of process switch.
  * We cannot take any locks, because we could be executing especially
  * within a kmmio critical section.
  *
  * Local interrupts are disabled, so preemption cannot happen.
  * Do not enable interrupts, do not sleep, and watch out for other CPUs.
  */
 /*
  * Interrupts are disabled on entry as trap3 is an interrupt gate
  * and they remain disabled throughout this function.
  */
 int kmmio_handler(struct pt_regs *regs, unsigned long addr)
 {
 	struct kmmio_context *ctx;
 	struct kmmio_fault_page *faultpage;
 	int ret = 0; /* default to fault not handled */

 	/*
 	 * Preemption is now disabled to prevent process switch during
 	 * single stepping. We can only handle one active kmmio trace
 	 * per cpu, so ensure that we finish it before something else
 	 * gets to run. We also hold the RCU read lock over single
 	 * stepping to avoid looking up the probe and kmmio_fault_page
 	 * again.
 	 */
 	preempt_disable();
 	rcu_read_lock();

 	faultpage = get_kmmio_fault_page(addr);
 	if (!faultpage) {
 		/*
 		 * Either this page fault is not caused by kmmio, or
 		 * another CPU just pulled the kmmio probe from under
 		 * our feet. The latter case should not be possible.
 		 */
 		goto no_kmmio;
 	}

 	ctx = &get_cpu_var(kmmio_ctx);
 	if (ctx->active) {
 		if (addr == ctx->addr) {
 			/*
 			 * A second fault on the same page means some other
 			 * condition needs handling by do_page_fault(), the
 			 * page really not being present is the most common.
 			 */
 			pr_debug("secondary hit for 0x%08lx CPU %d.\n",
 				 addr, smp_processor_id());

 			if (!faultpage->old_presence)
 				pr_info("unexpected secondary hit for address 0x%08lx on CPU %d.\n",
 					addr, smp_processor_id());
 		} else {
 			/*
 			 * Prevent overwriting already in-flight context.
 			 * This should not happen, let's hope disarming at
 			 * least prevents a panic.
 			 */
 			pr_emerg("recursive probe hit on CPU %d, for address 0x%08lx. Ignoring.\n",
 				 smp_processor_id(), addr);
 			pr_emerg("previous hit was at 0x%08lx.\n", ctx->addr);
 			disarm_kmmio_fault_page(faultpage);
 		}
 		goto no_kmmio_ctx;
 	}
 	ctx->active++;

 	ctx->fpage = faultpage;
 	ctx->probe = get_kmmio_probe(addr);
 	ctx->saved_flags = (regs->flags & (X86_EFLAGS_TF | X86_EFLAGS_IF));
 	ctx->addr = addr;

 	if (ctx->probe && ctx->probe->pre_handler)
 		ctx->probe->pre_handler(ctx->probe, regs, addr);

 	/*
 	 * Enable single-stepping and disable interrupts for the faulting
 	 * context. Local interrupts must not get enabled during stepping.
 	 */
 	regs->flags |= X86_EFLAGS_TF;
 	regs->flags &= ~X86_EFLAGS_IF;

 	/* Now we set present bit in PTE and single step. */
 	disarm_kmmio_fault_page(ctx->fpage);

 	/*
 	 * If another cpu accesses the same page while we are stepping,
 	 * the access will not be caught. It will simply succeed and the
 	 * only downside is we lose the event. If this becomes a problem,
 	 * the user should drop to single cpu before tracing.
 	 */

 	put_cpu_var(kmmio_ctx);
 	return 1; /* fault handled */

 no_kmmio_ctx:
 	put_cpu_var(kmmio_ctx);
 no_kmmio:
 	rcu_read_unlock();
 	preempt_enable_no_resched();
 	return ret;
 }

 /*
  * Interrupts are disabled on entry as trap1 is an interrupt gate
  * and they remain disabled throughout this function.
  * This must always get called as the pair to kmmio_handler().
  */
 static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
 {
 	int ret = 0;
 	struct kmmio_context *ctx = &get_cpu_var(kmmio_ctx);

 	if (!ctx->active) {
 		/*
 		 * debug traps without an active context are due to either
 		 * something external causing them (f.e. using a debugger while
 		 * mmio tracing enabled), or erroneous behaviour
 		 */
 		pr_warning("unexpected debug trap on CPU %d.\n",
 			   smp_processor_id());
 		goto out;
 	}

 	if (ctx->probe && ctx->probe->post_handler)
 		ctx->probe->post_handler(ctx->probe, condition, regs);

 	/* Prevent racing against release_kmmio_fault_page(). */
 	spin_lock(&kmmio_lock);
 	if (ctx->fpage->count)
 		arm_kmmio_fault_page(ctx->fpage);
 	spin_unlock(&kmmio_lock);

 	regs->flags &= ~X86_EFLAGS_TF;
 	regs->flags |= ctx->saved_flags;

 	/* These were acquired in kmmio_handler(). */
 	ctx->active--;
 	BUG_ON(ctx->active);
 	rcu_read_unlock();
 	preempt_enable_no_resched();

 	/*
 	 * if somebody else is singlestepping across a probe point, flags
 	 * will have TF set, in which case, continue the remaining processing
 	 * of do_debug, as if this is not a probe hit.
 	 */
 	if (!(regs->flags & X86_EFLAGS_TF))
 		ret = 1;
 out:
 	put_cpu_var(kmmio_ctx);
 	return ret;
 }

 /* You must be holding kmmio_lock. */
 static int add_kmmio_fault_page(unsigned long page)
 {
 	struct kmmio_fault_page *f;

 	page &= PAGE_MASK;
 	f = get_kmmio_fault_page(page);
 	if (f) {
 		if (!f->count)
 			arm_kmmio_fault_page(f);
 		f->count++;
 		return 0;
 	}

 	f = kzalloc(sizeof(*f), GFP_ATOMIC);
 	if (!f)
 		return -1;

 	f->count = 1;
 	f->page = page;

 	if (arm_kmmio_fault_page(f)) {
 		kfree(f);
 		return -1;
 	}

 	list_add_rcu(&f->list, kmmio_page_list(f->page));

 	return 0;
 }

 /* You must be holding kmmio_lock. */
 static void release_kmmio_fault_page(unsigned long page,
 				struct kmmio_fault_page **release_list)
 {
 	struct kmmio_fault_page *f;

 	page &= PAGE_MASK;
 	f = get_kmmio_fault_page(page);
 	if (!f)
 		return;

 	f->count--;
 	BUG_ON(f->count < 0);
 	if (!f->count) {
 		disarm_kmmio_fault_page(f);
 		f->release_next = *release_list;
 		*release_list = f;
 	}
 }

 /*
  * With page-unaligned ioremaps, one or two armed pages may contain
  * addresses from outside the intended mapping. Events for these addresses
  * are currently silently dropped. The events may result only from programming
  * mistakes by accessing addresses before the beginning or past the end of a
  * mapping.
  */
 int register_kmmio_probe(struct kmmio_probe *p)
 {
 	unsigned long flags;
 	int ret = 0;
 	unsigned long size = 0;
 	const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);

 	spin_lock_irqsave(&kmmio_lock, flags);
 	if (get_kmmio_probe(p->addr)) {
 		ret = -EEXIST;
 		goto out;
 	}
 	kmmio_count++;
 	list_add_rcu(&p->list, &kmmio_probes);
 	while (size < size_lim) {
 		if (add_kmmio_fault_page(p->addr + size))
 			pr_err("Unable to set page fault.\n");
 		size += PAGE_SIZE;
 	}
 out:
 	spin_unlock_irqrestore(&kmmio_lock, flags);
 	/*
 	 * XXX: What should I do here?
 	 * Here was a call to global_flush_tlb(), but it does not exist
 	 * anymore. It seems it's not needed after all.
 	 */
 	return ret;
 }
 EXPORT_SYMBOL(register_kmmio_probe);

 static void rcu_free_kmmio_fault_pages(struct rcu_head *head)
 {
 	struct kmmio_delayed_release *dr = container_of(
 						head,
 						struct kmmio_delayed_release,
 						rcu);
 	struct kmmio_fault_page *f = dr->release_list;
 	while (f) {
 		struct kmmio_fault_page *next = f->release_next;
 		BUG_ON(f->count);
 		kfree(f);
 		f = next;
 	}
 	kfree(dr);
 }

 static void remove_kmmio_fault_pages(struct rcu_head *head)
 {
 	struct kmmio_delayed_release *dr =
 		container_of(head, struct kmmio_delayed_release, rcu);
 	struct kmmio_fault_page *f = dr->release_list;
 	struct kmmio_fault_page **prevp = &dr->release_list;
 	unsigned long flags;

 	spin_lock_irqsave(&kmmio_lock, flags);
 	while (f) {
 		if (!f->count) {
 			list_del_rcu(&f->list);
 			prevp = &f->release_next;
 		} else {
 			*prevp = f->release_next;
 		}
 		f = f->release_next;
 	}
 	spin_unlock_irqrestore(&kmmio_lock, flags);

 	/* This is the real RCU destroy call. */
 	call_rcu(&dr->rcu, rcu_free_kmmio_fault_pages);
 }

 /*
  * Remove a kmmio probe. You have to synchronize_rcu() before you can be
  * sure that the callbacks will not be called anymore. Only after that
  * you may actually release your struct kmmio_probe.
  *
  * Unregistering a kmmio fault page has three steps:
  * 1. release_kmmio_fault_page()
  *    Disarm the page, wait a grace period to let all faults finish.
  * 2. remove_kmmio_fault_pages()
  *    Remove the pages from kmmio_page_table.
  * 3. rcu_free_kmmio_fault_pages()
  *    Actually free the kmmio_fault_page structs as with RCU.
  */
 void unregister_kmmio_probe(struct kmmio_probe *p)
 {
 	unsigned long flags;
 	unsigned long size = 0;
 	const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);
 	struct kmmio_fault_page *release_list = NULL;
 	struct kmmio_delayed_release *drelease;

 	spin_lock_irqsave(&kmmio_lock, flags);
 	while (size < size_lim) {
 		release_kmmio_fault_page(p->addr + size, &release_list);
 		size += PAGE_SIZE;
 	}
 	list_del_rcu(&p->list);
 	kmmio_count--;
 	spin_unlock_irqrestore(&kmmio_lock, flags);

 	drelease = kmalloc(sizeof(*drelease), GFP_ATOMIC);
 	if (!drelease) {
 		pr_crit("leaking kmmio_fault_page objects.\n");
 		return;
 	}
 	drelease->release_list = release_list;

 	/*
 	 * This is not really RCU here. We have just disarmed a set of
 	 * pages so that they cannot trigger page faults anymore. However,
 	 * we cannot remove the pages from kmmio_page_table,
 	 * because a probe hit might be in flight on another CPU. The
 	 * pages are collected into a list, and they will be removed from
 	 * kmmio_page_table when it is certain that no probe hit related to
 	 * these pages can be in flight. RCU grace period sounds like a
 	 * good choice.
 	 *
 	 * If we removed the pages too early, kmmio page fault handler might
 	 * not find the respective kmmio_fault_page and determine it's not
 	 * a kmmio fault, when it actually is. This would lead to madness.
 	 */
 	call_rcu(&drelease->rcu, remove_kmmio_fault_pages);
 }
 EXPORT_SYMBOL(unregister_kmmio_probe);

 static int
 kmmio_die_notifier(struct notifier_block *nb, unsigned long val, void *args)
 {
 	struct die_args *arg = args;

 	if (val == DIE_DEBUG && (arg->err & DR_STEP))
 		if (post_kmmio_handler(arg->err, arg->regs) == 1) {
 			/*
 			 * Reset the BS bit in dr6 (pointed by args->err) to
 			 * denote completion of processing
 			 */
 			(*(unsigned long *)ERR_PTR(arg->err)) &= ~DR_STEP;
 			return NOTIFY_STOP;
 		}

 	return NOTIFY_DONE;
 }

 static struct notifier_block nb_die = {
 	.notifier_call = kmmio_die_notifier
 };

 int kmmio_init(void)
 {
 	int i;

 	for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++)
 		INIT_LIST_HEAD(&kmmio_page_table[i]);

 	return register_die_notifier(&nb_die);
 }

 void kmmio_cleanup(void)
 {
 	int i;

 	unregister_die_notifier(&nb_die);
 	for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++) {
 		WARN_ONCE(!list_empty(&kmmio_page_table[i]),
 			KERN_ERR "kmmio_page_table not empty at cleanup, any further tracing will leak memory.\n");
 	}
 }
	/* Support for MMIO probes.
	* Benfit many code from kprobes
	* (C) 2002 Louis Zhuang <louis.zhuang@intel.com>.
	* 2007 Alexander Eichner
	* 2008 Pekka Paalanen <pq@iki.fi>
	*/

	#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

	#include <linux/list.h>
	#include <linux/rculist.h>
	#include <linux/spinlock.h>
	#include <linux/hash.h>
	#include <linux/init.h>
	#include <linux/module.h>
	#include <linux/kernel.h>
	#include <linux/uaccess.h>
	#include <linux/ptrace.h>
	#include <linux/preempt.h>
	#include <linux/percpu.h>
	#include <linux/kdebug.h>
	#include <linux/mutex.h>
	#include <linux/io.h>
	#include <asm/cacheflush.h>
	#include <asm/tlbflush.h>
	#include <linux/errno.h>
	#include <asm/debugreg.h>
	#include <linux/mmiotrace.h>

	#define KMMIO_PAGE_HASH_BITS 4
	#define KMMIO_PAGE_TABLE_SIZE (1 << KMMIO_PAGE_HASH_BITS)

	struct kmmio_fault_page {
	struct list_head list;
	struct kmmio_fault_page *release_next;
	unsigned long page; /* location of the fault page */
	pteval_t old_presence; /* page presence prior to arming */
	bool armed;

	/*
	* Number of times this page has been registered as a part
	* of a probe. If zero, page is disarmed and this may be freed.
	* Used only by writers (RCU) and post_kmmio_handler().
	* Protected by kmmio_lock, when linked into kmmio_page_table.
	*/
	int count;
	};

	struct kmmio_delayed_release {
	struct rcu_head rcu;
	struct kmmio_fault_page *release_list;
	};

	struct kmmio_context {
	struct kmmio_fault_page *fpage;
	struct kmmio_probe *probe;
	unsigned long saved_flags;
	unsigned long addr;
	int active;
	};

	static DEFINE_SPINLOCK(kmmio_lock);

	/* Protected by kmmio_lock */
	unsigned int kmmio_count;

	/* Read-protected by RCU, write-protected by kmmio_lock. */
	static struct list_head kmmio_page_table[KMMIO_PAGE_TABLE_SIZE];
	static LIST_HEAD(kmmio_probes);

	static struct list_head *kmmio_page_list(unsigned long page)
	{
	return &kmmio_page_table[hash_long(page, KMMIO_PAGE_HASH_BITS)];
	}

	/* Accessed per-cpu */
	static DEFINE_PER_CPU(struct kmmio_context, kmmio_ctx);

	/*
	* this is basically a dynamic stabbing problem:
	* Could use the existing prio tree code or
	* Possible better implementations:
	* The Interval Skip List: A Data Structure for Finding All Intervals That
	* Overlap a Point (might be simple)
	* Space Efficient Dynamic Stabbing with Fast Queries - Mikkel Thorup
	*/
	/* Get the kmmio at this addr (if any). You must be holding RCU read lock. */
	static struct kmmio_probe *get_kmmio_probe(unsigned long addr)
	{
	struct kmmio_probe *p;
	list_for_each_entry_rcu(p, &kmmio_probes, list) {
	if (addr >= p->addr && addr < (p->addr + p->len))
	return p;
	}
	return NULL;
	}

	/* You must be holding RCU read lock. */
	static struct kmmio_fault_page *get_kmmio_fault_page(unsigned long page)
	{
	struct list_head *head;
	struct kmmio_fault_page *f;

	page &= PAGE_MASK;
	head = kmmio_page_list(page);
	list_for_each_entry_rcu(f, head, list) {
	if (f->page == page)
	return f;
	}
	return NULL;
	}

	static void clear_pmd_presence(pmd_t pmd, bool clear, pmdval_t old)
	{
	pmdval_t v = pmd_val(*pmd);
	if (clear) {
	*old = v & _PAGE_PRESENT;
	v &= ~_PAGE_PRESENT;
	} else /* presume this has been called with clear==true previously */
	v \|= *old;
	set_pmd(pmd, __pmd(v));
	}

	static void clear_pte_presence(pte_t pte, bool clear, pteval_t old)
	{
	pteval_t v = pte_val(*pte);
	if (clear) {
	*old = v & _PAGE_PRESENT;
	v &= ~_PAGE_PRESENT;
	} else /* presume this has been called with clear==true previously */
	v \|= *old;
	set_pte_atomic(pte, __pte(v));
	}

	static int clear_page_presence(struct kmmio_fault_page *f, bool clear)
	{
	unsigned int level;
	pte_t *pte = lookup_address(f->page, &level);

	if (!pte) {
	pr_err("no pte for page 0x%08lx\n", f->page);
	return -1;
	}

	switch (level) {
	case PG_LEVEL_2M:
	clear_pmd_presence((pmd_t *)pte, clear, &f->old_presence);
	break;
	case PG_LEVEL_4K:
	clear_pte_presence(pte, clear, &f->old_presence);
	break;
	default:
	pr_err("unexpected page level 0x%x.\n", level);
	return -1;
	}

	__flush_tlb_one(f->page);
	return 0;
	}

	/*
	* Mark the given page as not present. Access to it will trigger a fault.
	*
	* Struct kmmio_fault_page is protected by RCU and kmmio_lock, but the
	* protection is ignored here. RCU read lock is assumed held, so the struct
	* will not disappear unexpectedly. Furthermore, the caller must guarantee,
	* that double arming the same virtual address (page) cannot occur.
	*
	* Double disarming on the other hand is allowed, and may occur when a fault
	* and mmiotrace shutdown happen simultaneously.
	*/
	static int arm_kmmio_fault_page(struct kmmio_fault_page *f)
	{
	int ret;
	WARN_ONCE(f->armed, KERN_ERR pr_fmt("kmmio page already armed.\n"));
	if (f->armed) {
	pr_warning("double-arm: page 0x%08lx, ref %d, old %d\n",
	f->page, f->count, !!f->old_presence);
	}
	ret = clear_page_presence(f, true);
	WARN_ONCE(ret < 0, KERN_ERR pr_fmt("arming 0x%08lx failed.\n"),
	f->page);
	f->armed = true;
	return ret;
	}

	/** Restore the given page to saved presence state. */
	static void disarm_kmmio_fault_page(struct kmmio_fault_page *f)
	{
	int ret = clear_page_presence(f, false);
	WARN_ONCE(ret < 0,
	KERN_ERR "kmmio disarming 0x%08lx failed.\n", f->page);
	f->armed = false;
	}

	/*
	* This is being called from do_page_fault().
	*
	* We may be in an interrupt or a critical section. Also prefecthing may
	* trigger a page fault. We may be in the middle of process switch.
	* We cannot take any locks, because we could be executing especially
	* within a kmmio critical section.
	*
	* Local interrupts are disabled, so preemption cannot happen.
	* Do not enable interrupts, do not sleep, and watch out for other CPUs.
	*/
	/*
	* Interrupts are disabled on entry as trap3 is an interrupt gate
	* and they remain disabled throughout this function.
	*/
	int kmmio_handler(struct pt_regs *regs, unsigned long addr)
	{
	struct kmmio_context *ctx;
	struct kmmio_fault_page *faultpage;
	int ret = 0; /* default to fault not handled */

	/*
	* Preemption is now disabled to prevent process switch during
	* single stepping. We can only handle one active kmmio trace
	* per cpu, so ensure that we finish it before something else
	* gets to run. We also hold the RCU read lock over single
	* stepping to avoid looking up the probe and kmmio_fault_page
	* again.
	*/
	preempt_disable();
	rcu_read_lock();

	faultpage = get_kmmio_fault_page(addr);
	if (!faultpage) {
	/*
	* Either this page fault is not caused by kmmio, or
	* another CPU just pulled the kmmio probe from under
	* our feet. The latter case should not be possible.
	*/
	goto no_kmmio;
	}

	ctx = &get_cpu_var(kmmio_ctx);
	if (ctx->active) {
	if (addr == ctx->addr) {
	/*
	* A second fault on the same page means some other
	* condition needs handling by do_page_fault(), the
	* page really not being present is the most common.
	*/
	pr_debug("secondary hit for 0x%08lx CPU %d.\n",
	addr, smp_processor_id());

	if (!faultpage->old_presence)
	pr_info("unexpected secondary hit for address 0x%08lx on CPU %d.\n",
	addr, smp_processor_id());
	} else {
	/*
	* Prevent overwriting already in-flight context.
	* This should not happen, let's hope disarming at
	* least prevents a panic.
	*/
	pr_emerg("recursive probe hit on CPU %d, for address 0x%08lx. Ignoring.\n",
	smp_processor_id(), addr);
	pr_emerg("previous hit was at 0x%08lx.\n", ctx->addr);
	disarm_kmmio_fault_page(faultpage);
	}
	goto no_kmmio_ctx;
	}
	ctx->active++;

	ctx->fpage = faultpage;
	ctx->probe = get_kmmio_probe(addr);
	ctx->saved_flags = (regs->flags & (X86_EFLAGS_TF \| X86_EFLAGS_IF));
	ctx->addr = addr;

	if (ctx->probe && ctx->probe->pre_handler)
	ctx->probe->pre_handler(ctx->probe, regs, addr);

	/*
	* Enable single-stepping and disable interrupts for the faulting
	* context. Local interrupts must not get enabled during stepping.
	*/
	regs->flags \|= X86_EFLAGS_TF;
	regs->flags &= ~X86_EFLAGS_IF;

	/* Now we set present bit in PTE and single step. */
	disarm_kmmio_fault_page(ctx->fpage);

	/*
	* If another cpu accesses the same page while we are stepping,
	* the access will not be caught. It will simply succeed and the
	* only downside is we lose the event. If this becomes a problem,
	* the user should drop to single cpu before tracing.
	*/

	put_cpu_var(kmmio_ctx);
	return 1; /* fault handled */

	no_kmmio_ctx:
	put_cpu_var(kmmio_ctx);
	no_kmmio:
	rcu_read_unlock();
	preempt_enable_no_resched();
	return ret;
	}

	/*
	* Interrupts are disabled on entry as trap1 is an interrupt gate
	* and they remain disabled throughout this function.
	* This must always get called as the pair to kmmio_handler().
	*/
	static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
	{
	int ret = 0;
	struct kmmio_context *ctx = &get_cpu_var(kmmio_ctx);

	if (!ctx->active) {
	/*
	* debug traps without an active context are due to either
	* something external causing them (f.e. using a debugger while
	* mmio tracing enabled), or erroneous behaviour
	*/
	pr_warning("unexpected debug trap on CPU %d.\n",
	smp_processor_id());
	goto out;
	}

	if (ctx->probe && ctx->probe->post_handler)
	ctx->probe->post_handler(ctx->probe, condition, regs);

	/* Prevent racing against release_kmmio_fault_page(). */
	spin_lock(&kmmio_lock);
	if (ctx->fpage->count)
	arm_kmmio_fault_page(ctx->fpage);
	spin_unlock(&kmmio_lock);

	regs->flags &= ~X86_EFLAGS_TF;
	regs->flags \|= ctx->saved_flags;

	/* These were acquired in kmmio_handler(). */
	ctx->active--;
	BUG_ON(ctx->active);
	rcu_read_unlock();
	preempt_enable_no_resched();

	/*
	* if somebody else is singlestepping across a probe point, flags
	* will have TF set, in which case, continue the remaining processing
	* of do_debug, as if this is not a probe hit.
	*/
	if (!(regs->flags & X86_EFLAGS_TF))
	ret = 1;
	out:
	put_cpu_var(kmmio_ctx);
	return ret;
	}

	/* You must be holding kmmio_lock. */
	static int add_kmmio_fault_page(unsigned long page)
	{
	struct kmmio_fault_page *f;

	page &= PAGE_MASK;
	f = get_kmmio_fault_page(page);
	if (f) {
	if (!f->count)
	arm_kmmio_fault_page(f);
	f->count++;
	return 0;
	}

	f = kzalloc(sizeof(*f), GFP_ATOMIC);
	if (!f)
	return -1;

	f->count = 1;
	f->page = page;

	if (arm_kmmio_fault_page(f)) {
	kfree(f);
	return -1;
	}

	list_add_rcu(&f->list, kmmio_page_list(f->page));

	return 0;
	}

	/* You must be holding kmmio_lock. */
	static void release_kmmio_fault_page(unsigned long page,
	struct kmmio_fault_page **release_list)
	{
	struct kmmio_fault_page *f;

	page &= PAGE_MASK;
	f = get_kmmio_fault_page(page);
	if (!f)
	return;

	f->count--;
	BUG_ON(f->count < 0);
	if (!f->count) {
	disarm_kmmio_fault_page(f);
	f->release_next = *release_list;
	*release_list = f;
	}
	}

	/*
	* With page-unaligned ioremaps, one or two armed pages may contain
	* addresses from outside the intended mapping. Events for these addresses
	* are currently silently dropped. The events may result only from programming
	* mistakes by accessing addresses before the beginning or past the end of a
	* mapping.
	*/
	int register_kmmio_probe(struct kmmio_probe *p)
	{
	unsigned long flags;
	int ret = 0;
	unsigned long size = 0;
	const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);

	spin_lock_irqsave(&kmmio_lock, flags);
	if (get_kmmio_probe(p->addr)) {
	ret = -EEXIST;
	goto out;
	}
	kmmio_count++;
	list_add_rcu(&p->list, &kmmio_probes);
	while (size < size_lim) {
	if (add_kmmio_fault_page(p->addr + size))
	pr_err("Unable to set page fault.\n");
	size += PAGE_SIZE;
	}
	out:
	spin_unlock_irqrestore(&kmmio_lock, flags);
	/*
	* XXX: What should I do here?
	* Here was a call to global_flush_tlb(), but it does not exist
	* anymore. It seems it's not needed after all.
	*/
	return ret;
	}
	EXPORT_SYMBOL(register_kmmio_probe);

	static void rcu_free_kmmio_fault_pages(struct rcu_head *head)
	{
	struct kmmio_delayed_release *dr = container_of(
	head,
	struct kmmio_delayed_release,
	rcu);
	struct kmmio_fault_page *f = dr->release_list;
	while (f) {
	struct kmmio_fault_page *next = f->release_next;
	BUG_ON(f->count);
	kfree(f);
	f = next;
	}
	kfree(dr);
	}

	static void remove_kmmio_fault_pages(struct rcu_head *head)
	{
	struct kmmio_delayed_release *dr =
	container_of(head, struct kmmio_delayed_release, rcu);
	struct kmmio_fault_page *f = dr->release_list;
	struct kmmio_fault_page **prevp = &dr->release_list;
	unsigned long flags;

	spin_lock_irqsave(&kmmio_lock, flags);
	while (f) {
	if (!f->count) {
	list_del_rcu(&f->list);
	prevp = &f->release_next;
	} else {
	*prevp = f->release_next;
	}
	f = f->release_next;
	}
	spin_unlock_irqrestore(&kmmio_lock, flags);

	/* This is the real RCU destroy call. */
	call_rcu(&dr->rcu, rcu_free_kmmio_fault_pages);
	}

	/*
	* Remove a kmmio probe. You have to synchronize_rcu() before you can be
	* sure that the callbacks will not be called anymore. Only after that
	* you may actually release your struct kmmio_probe.
	*
	* Unregistering a kmmio fault page has three steps:
	* 1. release_kmmio_fault_page()
	* Disarm the page, wait a grace period to let all faults finish.
	* 2. remove_kmmio_fault_pages()
	* Remove the pages from kmmio_page_table.
	* 3. rcu_free_kmmio_fault_pages()
	* Actually free the kmmio_fault_page structs as with RCU.
	*/
	void unregister_kmmio_probe(struct kmmio_probe *p)
	{
	unsigned long flags;
	unsigned long size = 0;
	const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);
	struct kmmio_fault_page *release_list = NULL;
	struct kmmio_delayed_release *drelease;

	spin_lock_irqsave(&kmmio_lock, flags);
	while (size < size_lim) {
	release_kmmio_fault_page(p->addr + size, &release_list);
	size += PAGE_SIZE;
	}
	list_del_rcu(&p->list);
	kmmio_count--;
	spin_unlock_irqrestore(&kmmio_lock, flags);

	drelease = kmalloc(sizeof(*drelease), GFP_ATOMIC);
	if (!drelease) {
	pr_crit("leaking kmmio_fault_page objects.\n");
	return;
	}
	drelease->release_list = release_list;

	/*
	* This is not really RCU here. We have just disarmed a set of
	* pages so that they cannot trigger page faults anymore. However,
	* we cannot remove the pages from kmmio_page_table,
	* because a probe hit might be in flight on another CPU. The
	* pages are collected into a list, and they will be removed from
	* kmmio_page_table when it is certain that no probe hit related to
	* these pages can be in flight. RCU grace period sounds like a
	* good choice.
	*
	* If we removed the pages too early, kmmio page fault handler might
	* not find the respective kmmio_fault_page and determine it's not
	* a kmmio fault, when it actually is. This would lead to madness.
	*/
	call_rcu(&drelease->rcu, remove_kmmio_fault_pages);
	}
	EXPORT_SYMBOL(unregister_kmmio_probe);

	static int
	kmmio_die_notifier(struct notifier_block nb, unsigned long val, void args)
	{
	struct die_args *arg = args;

	if (val == DIE_DEBUG && (arg->err & DR_STEP))
	if (post_kmmio_handler(arg->err, arg->regs) == 1) {
	/*
	* Reset the BS bit in dr6 (pointed by args->err) to
	* denote completion of processing
	*/
	((unsigned long )ERR_PTR(arg->err)) &= ~DR_STEP;
	return NOTIFY_STOP;
	}

	return NOTIFY_DONE;
	}

	static struct notifier_block nb_die = {
	.notifier_call = kmmio_die_notifier
	};

	int kmmio_init(void)
	{
	int i;

	for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++)
	INIT_LIST_HEAD(&kmmio_page_table[i]);

	return register_die_notifier(&nb_die);
	}

	void kmmio_cleanup(void)
	{
	int i;

	unregister_die_notifier(&nb_die);
	for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++) {
	WARN_ONCE(!list_empty(&kmmio_page_table[i]),
	KERN_ERR "kmmio_page_table not empty at cleanup, any further tracing will leak memory.\n");
	}
	}