/*
 * 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>
#include <linux/kprobes.h>

#define CREATE_TRACE_POINTS
#include <trace/events/context_tracking.h>

DEFINE_STATIC_KEY_FALSE(context_tracking_enabled);
EXPORT_SYMBOL_GPL(context_tracking_enabled);

DEFINE_PER_CPU(struct context_tracking, context_tracking);
EXPORT_SYMBOL_GPL(context_tracking);

static bool context_tracking_recursion_enter(void)
{
        int recursion;

        recursion = __this_cpu_inc_return(context_tracking.recursion);
        if (recursion == 1)
                return true;

        WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion);
        __this_cpu_dec(context_tracking.recursion);

        return false;
}

static void context_tracking_recursion_exit(void)
{
        __this_cpu_dec(context_tracking.recursion);
}

/**
 * context_tracking_enter - Inform the context tracking that the CPU is going
 *                          enter user or guest space mode.
 *
 * This function must be called right before we switch from the kernel
 * to user or guest space, 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 __context_tracking_enter(enum ctx_state state)
{
        /* Kernel threads aren't supposed to go to userspace */
        WARN_ON_ONCE(!current->mm);

        if (!context_tracking_recursion_enter())
                return;

        if ( __this_cpu_read(context_tracking.state) != state) {
                if (__this_cpu_read(context_tracking.active)) {
                        /*
                         * 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.
                         */
                        if (state == CONTEXT_USER) {
                                trace_user_enter(0);
                                vtime_user_enter(current);
                        }
                        rcu_user_enter();
                }
                /*
                 * Even if context tracking is disabled on this CPU, because it's outside
                 * the full dynticks mask for example, we still have to keep track of the
                 * context transitions and states to prevent inconsistency on those of
                 * other CPUs.
                 * If a task triggers an exception in userspace, sleep on the exception
                 * handler and then migrate to another CPU, that new CPU must know where
                 * the exception returns by the time we call exception_exit().
                 * This information can only be provided by the previous CPU when it called
                 * exception_enter().
                 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
                 * is false because we know that CPU is not tickless.
                 */
                __this_cpu_write(context_tracking.state, state);
        }
        context_tracking_recursion_exit();
}
NOKPROBE_SYMBOL(__context_tracking_enter);
EXPORT_SYMBOL_GPL(__context_tracking_enter);

void context_tracking_enter(enum ctx_state state)
{
        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;

        local_irq_save(flags);
        __context_tracking_enter(state);
        local_irq_restore(flags);
}
NOKPROBE_SYMBOL(context_tracking_enter);
EXPORT_SYMBOL_GPL(context_tracking_enter);

void context_tracking_user_enter(void)
{
        user_enter();
}
NOKPROBE_SYMBOL(context_tracking_user_enter);

/**
 * context_tracking_exit - Inform the context tracking that the CPU is
 *                         exiting user or guest mode and entering the kernel.
 *
 * This function must be called after we entered the kernel from user or
 * guest space 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 __context_tracking_exit(enum ctx_state state)
{
        if (!context_tracking_recursion_enter())
                return;

        if (__this_cpu_read(context_tracking.state) == state) {
                if (__this_cpu_read(context_tracking.active)) {
                        /*
                         * 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();
                        if (state == CONTEXT_USER) {
                                vtime_user_exit(current);
                                trace_user_exit(0);
                        }
                }
                __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
        }
        context_tracking_recursion_exit();
}
NOKPROBE_SYMBOL(__context_tracking_exit);
EXPORT_SYMBOL_GPL(__context_tracking_exit);

void context_tracking_exit(enum ctx_state state)
{
        unsigned long flags;

        if (in_interrupt())
                return;

        local_irq_save(flags);
        __context_tracking_exit(state);
        local_irq_restore(flags);
}
NOKPROBE_SYMBOL(context_tracking_exit);
EXPORT_SYMBOL_GPL(context_tracking_exit);

void context_tracking_user_exit(void)
{
        user_exit();
}
NOKPROBE_SYMBOL(context_tracking_user_exit);

void __init context_tracking_cpu_set(int cpu)
{
        static __initdata bool initialized = false;

        if (!per_cpu(context_tracking.active, cpu)) {
                per_cpu(context_tracking.active, cpu) = true;
                static_branch_inc(&context_tracking_enabled);
        }

        if (initialized)
                return;

        /*
         * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork
         * This assumes that init is the only task at this early boot stage.
         */
        set_tsk_thread_flag(&init_task, TIF_NOHZ);
        WARN_ON_ONCE(!tasklist_empty());

        initialized = true;
}

#ifdef CONFIG_CONTEXT_TRACKING_FORCE
void __init context_tracking_init(void)
{
        int cpu;

        for_each_possible_cpu(cpu)
                context_tracking_cpu_set(cpu);
}
#endif