/* * 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/kvm_host.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);}  /** * 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);	}}