/*
   * Context tracking: Probe on high level context boundaries such as kernel
   * and userspace. This includes syscalls and exceptions entry/exit.
   *
   * This is used by RCU to remove its dependency on the timer tick while a CPU
   * runs in userspace.
   *
   *  Started by Frederic Weisbecker:
   *
   * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
   *
   * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
   * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
   *
   */

  #include <linux/context_tracking.h>
  #include <linux/rcupdate.h>
  #include <linux/sched.h>

  #include <linux/hardirq.h>

  #include <linux/export.h>

  DEFINE_PER_CPU(struct context_tracking, context_tracking) = {

  #ifdef CONFIG_CONTEXT_TRACKING_FORCE
  	.active = true,
  #endif
  };

  /**
   * user_enter - Inform the context tracking that the CPU is going to
   *              enter userspace mode.
   *
   * This function must be called right before we switch from the kernel
   * to userspace, when it's guaranteed the remaining kernel instructions
   * to execute won't use any RCU read side critical section because this
   * function sets RCU in extended quiescent state.
   */

  void user_enter(void)
  {
  	unsigned long flags;
  
  	/*
  	 * Some contexts may involve an exception occuring in an irq,
  	 * leading to that nesting:
  	 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
  	 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
  	 * helpers are enough to protect RCU uses inside the exception. So
  	 * just return immediately if we detect we are in an IRQ.
  	 */
  	if (in_interrupt())
  		return;

  	/* Kernel threads aren't supposed to go to userspace */

  	WARN_ON_ONCE(!current->mm);
  
  	local_irq_save(flags);
  	if (__this_cpu_read(context_tracking.active) &&
  	    __this_cpu_read(context_tracking.state) != IN_USER) {

  		/*
  		 * At this stage, only low level arch entry code remains and
  		 * then we'll run in userspace. We can assume there won't be
  		 * any RCU read-side critical section until the next call to
  		 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
  		 * on the tick.
  		 */

  		vtime_user_enter(current);

  		rcu_user_enter();

  		__this_cpu_write(context_tracking.state, IN_USER);

  	}
  	local_irq_restore(flags);
  }

  #ifdef CONFIG_PREEMPT
  /**
   * preempt_schedule_context - preempt_schedule called by tracing
   *
   * The tracing infrastructure uses preempt_enable_notrace to prevent
   * recursion and tracing preempt enabling caused by the tracing
   * infrastructure itself. But as tracing can happen in areas coming
   * from userspace or just about to enter userspace, a preempt enable
   * can occur before user_exit() is called. This will cause the scheduler
   * to be called when the system is still in usermode.
   *
   * To prevent this, the preempt_enable_notrace will use this function
   * instead of preempt_schedule() to exit user context if needed before
   * calling the scheduler.
   */
  void __sched notrace preempt_schedule_context(void)
  {
  	struct thread_info *ti = current_thread_info();
  	enum ctx_state prev_ctx;
  
  	if (likely(ti->preempt_count || irqs_disabled()))
  		return;
  
  	/*
  	 * Need to disable preemption in case user_exit() is traced
  	 * and the tracer calls preempt_enable_notrace() causing
  	 * an infinite recursion.
  	 */
  	preempt_disable_notrace();
  	prev_ctx = exception_enter();
  	preempt_enable_no_resched_notrace();
  
  	preempt_schedule();
  
  	preempt_disable_notrace();
  	exception_exit(prev_ctx);
  	preempt_enable_notrace();
  }
  EXPORT_SYMBOL_GPL(preempt_schedule_context);
  #endif /* CONFIG_PREEMPT */

  
  /**
   * user_exit - Inform the context tracking that the CPU is
   *             exiting userspace mode and entering the kernel.
   *
   * This function must be called after we entered the kernel from userspace
   * before any use of RCU read side critical section. This potentially include
   * any high level kernel code like syscalls, exceptions, signal handling, etc...
   *
   * This call supports re-entrancy. This way it can be called from any exception
   * handler without needing to know if we came from userspace or not.
   */

  void user_exit(void)
  {
  	unsigned long flags;

  	if (in_interrupt())
  		return;
  
  	local_irq_save(flags);
  	if (__this_cpu_read(context_tracking.state) == IN_USER) {

  		/*
  		 * We are going to run code that may use RCU. Inform
  		 * RCU core about that (ie: we may need the tick again).
  		 */

  		rcu_user_exit();

  		vtime_user_exit(current);
  		__this_cpu_write(context_tracking.state, IN_KERNEL);

  	}
  	local_irq_restore(flags);
  }

  void guest_enter(void)
  {
  	if (vtime_accounting_enabled())
  		vtime_guest_enter(current);
  	else
  		__guest_enter();
  }
  EXPORT_SYMBOL_GPL(guest_enter);
  
  void guest_exit(void)
  {
  	if (vtime_accounting_enabled())
  		vtime_guest_exit(current);
  	else
  		__guest_exit();
  }
  EXPORT_SYMBOL_GPL(guest_exit);

  
  /**
   * context_tracking_task_switch - context switch the syscall callbacks
   * @prev: the task that is being switched out
   * @next: the task that is being switched in
   *
   * The context tracking uses the syscall slow path to implement its user-kernel
   * boundaries probes on syscalls. This way it doesn't impact the syscall fast
   * path on CPUs that don't do context tracking.
   *
   * But we need to clear the flag on the previous task because it may later
   * migrate to some CPU that doesn't do the context tracking. As such the TIF
   * flag may not be desired there.
   */

  void context_tracking_task_switch(struct task_struct *prev,
  			     struct task_struct *next)
  {
  	if (__this_cpu_read(context_tracking.active)) {
  		clear_tsk_thread_flag(prev, TIF_NOHZ);
  		set_tsk_thread_flag(next, TIF_NOHZ);
  	}
  }