KVM: x86/speculation: Disable Fill buffer clear within guests

[mirror_ubuntu-jammy-kernel.git] / kernel / stop_machine.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * kernel/stop_machine.c
 *
 * Copyright (C) 2008, 2005     IBM Corporation.
 * Copyright (C) 2008, 2005     Rusty Russell rusty@rustcorp.com.au
 * Copyright (C) 2010           SUSE Linux Products GmbH
 * Copyright (C) 2010           Tejun Heo <tj@kernel.org>
 */
#include <linux/compiler.h>
#include <linux/completion.h>
#include <linux/cpu.h>
#include <linux/init.h>
#include <linux/kthread.h>
#include <linux/export.h>
#include <linux/percpu.h>
#include <linux/sched.h>
#include <linux/stop_machine.h>
#include <linux/interrupt.h>
#include <linux/kallsyms.h>
#include <linux/smpboot.h>
#include <linux/atomic.h>
#include <linux/nmi.h>
#include <linux/sched/wake_q.h>

/*
 * Structure to determine completion condition and record errors.  May
 * be shared by works on different cpus.
 */
struct cpu_stop_done {
        atomic_t                nr_todo;        /* nr left to execute */
        int                     ret;            /* collected return value */
        struct completion       completion;     /* fired if nr_todo reaches 0 */
};

/* the actual stopper, one per every possible cpu, enabled on online cpus */
struct cpu_stopper {
        struct task_struct      *thread;

        raw_spinlock_t          lock;
        bool                    enabled;        /* is this stopper enabled? */
        struct list_head        works;          /* list of pending works */

        struct cpu_stop_work    stop_work;      /* for stop_cpus */
        unsigned long           caller;
        cpu_stop_fn_t           fn;
};

static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);
static bool stop_machine_initialized = false;

void print_stop_info(const char *log_lvl, struct task_struct *task)
{
        /*
         * If @task is a stopper task, it cannot migrate and task_cpu() is
         * stable.
         */
        struct cpu_stopper *stopper = per_cpu_ptr(&cpu_stopper, task_cpu(task));

        if (task != stopper->thread)
                return;

        printk("%sStopper: %pS <- %pS\n", log_lvl, stopper->fn, (void *)stopper->caller);
}

/* static data for stop_cpus */
static DEFINE_MUTEX(stop_cpus_mutex);
static bool stop_cpus_in_progress;

static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
{
        memset(done, 0, sizeof(*done));
        atomic_set(&done->nr_todo, nr_todo);
        init_completion(&done->completion);
}

/* signal completion unless @done is NULL */
static void cpu_stop_signal_done(struct cpu_stop_done *done)
{
        if (atomic_dec_and_test(&done->nr_todo))
                complete(&done->completion);
}

static void __cpu_stop_queue_work(struct cpu_stopper *stopper,
                                        struct cpu_stop_work *work,
                                        struct wake_q_head *wakeq)
{
        list_add_tail(&work->list, &stopper->works);
        wake_q_add(wakeq, stopper->thread);
}

/* queue @work to @stopper.  if offline, @work is completed immediately */
static bool cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
        DEFINE_WAKE_Q(wakeq);
        unsigned long flags;
        bool enabled;

        preempt_disable();
        raw_spin_lock_irqsave(&stopper->lock, flags);
        enabled = stopper->enabled;
        if (enabled)
                __cpu_stop_queue_work(stopper, work, &wakeq);
        else if (work->done)
                cpu_stop_signal_done(work->done);
        raw_spin_unlock_irqrestore(&stopper->lock, flags);

        wake_up_q(&wakeq);
        preempt_enable();

        return enabled;
}

/**
 * stop_one_cpu - stop a cpu
 * @cpu: cpu to stop
 * @fn: function to execute
 * @arg: argument to @fn
 *
 * Execute @fn(@arg) on @cpu.  @fn is run in a process context with
 * the highest priority preempting any task on the cpu and
 * monopolizing it.  This function returns after the execution is
 * complete.
 *
 * This function doesn't guarantee @cpu stays online till @fn
 * completes.  If @cpu goes down in the middle, execution may happen
 * partially or fully on different cpus.  @fn should either be ready
 * for that or the caller should ensure that @cpu stays online until
 * this function completes.
 *
 * CONTEXT:
 * Might sleep.
 *
 * RETURNS:
 * -ENOENT if @fn(@arg) was not executed because @cpu was offline;
 * otherwise, the return value of @fn.
 */
int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
{
        struct cpu_stop_done done;
        struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = &done, .caller = _RET_IP_ };

        cpu_stop_init_done(&done, 1);
        if (!cpu_stop_queue_work(cpu, &work))
                return -ENOENT;
        /*
         * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
         * cycle by doing a preemption:
         */
        cond_resched();
        wait_for_completion(&done.completion);
        return done.ret;
}

/* This controls the threads on each CPU. */
enum multi_stop_state {
        /* Dummy starting state for thread. */
        MULTI_STOP_NONE,
        /* Awaiting everyone to be scheduled. */
        MULTI_STOP_PREPARE,
        /* Disable interrupts. */
        MULTI_STOP_DISABLE_IRQ,
        /* Run the function */
        MULTI_STOP_RUN,
        /* Exit */
        MULTI_STOP_EXIT,
};

struct multi_stop_data {
        cpu_stop_fn_t           fn;
        void                    *data;
        /* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
        unsigned int            num_threads;
        const struct cpumask    *active_cpus;

        enum multi_stop_state   state;
        atomic_t                thread_ack;
};

static void set_state(struct multi_stop_data *msdata,
                      enum multi_stop_state newstate)
{
        /* Reset ack counter. */
        atomic_set(&msdata->thread_ack, msdata->num_threads);
        smp_wmb();
        WRITE_ONCE(msdata->state, newstate);
}

/* Last one to ack a state moves to the next state. */
static void ack_state(struct multi_stop_data *msdata)
{
        if (atomic_dec_and_test(&msdata->thread_ack))
                set_state(msdata, msdata->state + 1);
}

notrace void __weak stop_machine_yield(const struct cpumask *cpumask)
{
        cpu_relax();
}

/* This is the cpu_stop function which stops the CPU. */
static int multi_cpu_stop(void *data)
{
        struct multi_stop_data *msdata = data;
        enum multi_stop_state newstate, curstate = MULTI_STOP_NONE;
        int cpu = smp_processor_id(), err = 0;
        const struct cpumask *cpumask;
        unsigned long flags;
        bool is_active;

        /*
         * When called from stop_machine_from_inactive_cpu(), irq might
         * already be disabled.  Save the state and restore it on exit.
         */
        local_save_flags(flags);

        if (!msdata->active_cpus) {
                cpumask = cpu_online_mask;
                is_active = cpu == cpumask_first(cpumask);
        } else {
                cpumask = msdata->active_cpus;
                is_active = cpumask_test_cpu(cpu, cpumask);
        }

        /* Simple state machine */
        do {
                /* Chill out and ensure we re-read multi_stop_state. */
                stop_machine_yield(cpumask);
                newstate = READ_ONCE(msdata->state);
                if (newstate != curstate) {
                        curstate = newstate;
                        switch (curstate) {
                        case MULTI_STOP_DISABLE_IRQ:
                                local_irq_disable();
                                hard_irq_disable();
                                break;
                        case MULTI_STOP_RUN:
                                if (is_active)
                                        err = msdata->fn(msdata->data);
                                break;
                        default:
                                break;
                        }
                        ack_state(msdata);
                } else if (curstate > MULTI_STOP_PREPARE) {
                        /*
                         * At this stage all other CPUs we depend on must spin
                         * in the same loop. Any reason for hard-lockup should
                         * be detected and reported on their side.
                         */
                        touch_nmi_watchdog();
                }
                rcu_momentary_dyntick_idle();
        } while (curstate != MULTI_STOP_EXIT);

        local_irq_restore(flags);
        return err;
}

static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
                                    int cpu2, struct cpu_stop_work *work2)
{
        struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
        struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
        DEFINE_WAKE_Q(wakeq);
        int err;

retry:
        /*
         * The waking up of stopper threads has to happen in the same
         * scheduling context as the queueing.  Otherwise, there is a
         * possibility of one of the above stoppers being woken up by another
         * CPU, and preempting us. This will cause us to not wake up the other
         * stopper forever.
         */
        preempt_disable();
        raw_spin_lock_irq(&stopper1->lock);
        raw_spin_lock_nested(&stopper2->lock, SINGLE_DEPTH_NESTING);

        if (!stopper1->enabled || !stopper2->enabled) {
                err = -ENOENT;
                goto unlock;
        }

        /*
         * Ensure that if we race with __stop_cpus() the stoppers won't get
         * queued up in reverse order leading to system deadlock.
         *
         * We can't miss stop_cpus_in_progress if queue_stop_cpus_work() has
         * queued a work on cpu1 but not on cpu2, we hold both locks.
         *
         * It can be falsely true but it is safe to spin until it is cleared,
         * queue_stop_cpus_work() does everything under preempt_disable().
         */
        if (unlikely(stop_cpus_in_progress)) {
                err = -EDEADLK;
                goto unlock;
        }

        err = 0;
        __cpu_stop_queue_work(stopper1, work1, &wakeq);
        __cpu_stop_queue_work(stopper2, work2, &wakeq);

unlock:
        raw_spin_unlock(&stopper2->lock);
        raw_spin_unlock_irq(&stopper1->lock);

        if (unlikely(err == -EDEADLK)) {
                preempt_enable();

                while (stop_cpus_in_progress)
                        cpu_relax();

                goto retry;
        }

        wake_up_q(&wakeq);
        preempt_enable();

        return err;
}
/**
 * stop_two_cpus - stops two cpus
 * @cpu1: the cpu to stop
 * @cpu2: the other cpu to stop
 * @fn: function to execute
 * @arg: argument to @fn
 *
 * Stops both the current and specified CPU and runs @fn on one of them.
 *
 * returns when both are completed.
 */
int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
{
        struct cpu_stop_done done;
        struct cpu_stop_work work1, work2;
        struct multi_stop_data msdata;

        msdata = (struct multi_stop_data){
                .fn = fn,
                .data = arg,
                .num_threads = 2,
                .active_cpus = cpumask_of(cpu1),
        };

        work1 = work2 = (struct cpu_stop_work){
                .fn = multi_cpu_stop,
                .arg = &msdata,
                .done = &done,
                .caller = _RET_IP_,
        };

        cpu_stop_init_done(&done, 2);
        set_state(&msdata, MULTI_STOP_PREPARE);

        if (cpu1 > cpu2)
                swap(cpu1, cpu2);
        if (cpu_stop_queue_two_works(cpu1, &work1, cpu2, &work2))
                return -ENOENT;

        wait_for_completion(&done.completion);
        return done.ret;
}

/**
 * stop_one_cpu_nowait - stop a cpu but don't wait for completion
 * @cpu: cpu to stop
 * @fn: function to execute
 * @arg: argument to @fn
 * @work_buf: pointer to cpu_stop_work structure
 *
 * Similar to stop_one_cpu() but doesn't wait for completion.  The
 * caller is responsible for ensuring @work_buf is currently unused
 * and will remain untouched until stopper starts executing @fn.
 *
 * CONTEXT:
 * Don't care.
 *
 * RETURNS:
 * true if cpu_stop_work was queued successfully and @fn will be called,
 * false otherwise.
 */
bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
                        struct cpu_stop_work *work_buf)
{
        *work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, .caller = _RET_IP_, };
        return cpu_stop_queue_work(cpu, work_buf);
}

static bool queue_stop_cpus_work(const struct cpumask *cpumask,
                                 cpu_stop_fn_t fn, void *arg,
                                 struct cpu_stop_done *done)
{
        struct cpu_stop_work *work;
        unsigned int cpu;
        bool queued = false;

        /*
         * Disable preemption while queueing to avoid getting
         * preempted by a stopper which might wait for other stoppers
         * to enter @fn which can lead to deadlock.
         */
        preempt_disable();
        stop_cpus_in_progress = true;
        barrier();
        for_each_cpu(cpu, cpumask) {
                work = &per_cpu(cpu_stopper.stop_work, cpu);
                work->fn = fn;
                work->arg = arg;
                work->done = done;
                work->caller = _RET_IP_;
                if (cpu_stop_queue_work(cpu, work))
                        queued = true;
        }
        barrier();
        stop_cpus_in_progress = false;
        preempt_enable();

        return queued;
}

static int __stop_cpus(const struct cpumask *cpumask,
                       cpu_stop_fn_t fn, void *arg)
{
        struct cpu_stop_done done;

        cpu_stop_init_done(&done, cpumask_weight(cpumask));
        if (!queue_stop_cpus_work(cpumask, fn, arg, &done))
                return -ENOENT;
        wait_for_completion(&done.completion);
        return done.ret;
}

/**
 * stop_cpus - stop multiple cpus
 * @cpumask: cpus to stop
 * @fn: function to execute
 * @arg: argument to @fn
 *
 * Execute @fn(@arg) on online cpus in @cpumask.  On each target cpu,
 * @fn is run in a process context with the highest priority
 * preempting any task on the cpu and monopolizing it.  This function
 * returns after all executions are complete.
 *
 * This function doesn't guarantee the cpus in @cpumask stay online
 * till @fn completes.  If some cpus go down in the middle, execution
 * on the cpu may happen partially or fully on different cpus.  @fn
 * should either be ready for that or the caller should ensure that
 * the cpus stay online until this function completes.
 *
 * All stop_cpus() calls are serialized making it safe for @fn to wait
 * for all cpus to start executing it.
 *
 * CONTEXT:
 * Might sleep.
 *
 * RETURNS:
 * -ENOENT if @fn(@arg) was not executed at all because all cpus in
 * @cpumask were offline; otherwise, 0 if all executions of @fn
 * returned 0, any non zero return value if any returned non zero.
 */
static int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
{
        int ret;

        /* static works are used, process one request at a time */
        mutex_lock(&stop_cpus_mutex);
        ret = __stop_cpus(cpumask, fn, arg);
        mutex_unlock(&stop_cpus_mutex);
        return ret;
}

static int cpu_stop_should_run(unsigned int cpu)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
        unsigned long flags;
        int run;

        raw_spin_lock_irqsave(&stopper->lock, flags);
        run = !list_empty(&stopper->works);
        raw_spin_unlock_irqrestore(&stopper->lock, flags);
        return run;
}

static void cpu_stopper_thread(unsigned int cpu)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
        struct cpu_stop_work *work;

repeat:
        work = NULL;
        raw_spin_lock_irq(&stopper->lock);
        if (!list_empty(&stopper->works)) {
                work = list_first_entry(&stopper->works,
                                        struct cpu_stop_work, list);
                list_del_init(&work->list);
        }
        raw_spin_unlock_irq(&stopper->lock);

        if (work) {
                cpu_stop_fn_t fn = work->fn;
                void *arg = work->arg;
                struct cpu_stop_done *done = work->done;
                int ret;

                /* cpu stop callbacks must not sleep, make in_atomic() == T */
                stopper->caller = work->caller;
                stopper->fn = fn;
                preempt_count_inc();
                ret = fn(arg);
                if (done) {
                        if (ret)
                                done->ret = ret;
                        cpu_stop_signal_done(done);
                }
                preempt_count_dec();
                stopper->fn = NULL;
                stopper->caller = 0;
                WARN_ONCE(preempt_count(),
                          "cpu_stop: %ps(%p) leaked preempt count\n", fn, arg);
                goto repeat;
        }
}

void stop_machine_park(int cpu)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
        /*
         * Lockless. cpu_stopper_thread() will take stopper->lock and flush
         * the pending works before it parks, until then it is fine to queue
         * the new works.
         */
        stopper->enabled = false;
        kthread_park(stopper->thread);
}

extern void sched_set_stop_task(int cpu, struct task_struct *stop);

static void cpu_stop_create(unsigned int cpu)
{
        sched_set_stop_task(cpu, per_cpu(cpu_stopper.thread, cpu));
}

static void cpu_stop_park(unsigned int cpu)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

        WARN_ON(!list_empty(&stopper->works));
}

void stop_machine_unpark(int cpu)
{
        struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

        stopper->enabled = true;
        kthread_unpark(stopper->thread);
}

static struct smp_hotplug_thread cpu_stop_threads = {
        .store                  = &cpu_stopper.thread,
        .thread_should_run      = cpu_stop_should_run,
        .thread_fn              = cpu_stopper_thread,
        .thread_comm            = "migration/%u",
        .create                 = cpu_stop_create,
        .park                   = cpu_stop_park,
        .selfparking            = true,
};

static int __init cpu_stop_init(void)
{
        unsigned int cpu;

        for_each_possible_cpu(cpu) {
                struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

                raw_spin_lock_init(&stopper->lock);
                INIT_LIST_HEAD(&stopper->works);
        }

        BUG_ON(smpboot_register_percpu_thread(&cpu_stop_threads));
        stop_machine_unpark(raw_smp_processor_id());
        stop_machine_initialized = true;
        return 0;
}
early_initcall(cpu_stop_init);

int stop_machine_cpuslocked(cpu_stop_fn_t fn, void *data,
                            const struct cpumask *cpus)
{
        struct multi_stop_data msdata = {
                .fn = fn,
                .data = data,
                .num_threads = num_online_cpus(),
                .active_cpus = cpus,
        };

        lockdep_assert_cpus_held();

        if (!stop_machine_initialized) {
                /*
                 * Handle the case where stop_machine() is called
                 * early in boot before stop_machine() has been
                 * initialized.
                 */
                unsigned long flags;
                int ret;

                WARN_ON_ONCE(msdata.num_threads != 1);

                local_irq_save(flags);
                hard_irq_disable();
                ret = (*fn)(data);
                local_irq_restore(flags);

                return ret;
        }

        /* Set the initial state and stop all online cpus. */
        set_state(&msdata, MULTI_STOP_PREPARE);
        return stop_cpus(cpu_online_mask, multi_cpu_stop, &msdata);
}

int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus)
{
        int ret;

        /* No CPUs can come up or down during this. */
        cpus_read_lock();
        ret = stop_machine_cpuslocked(fn, data, cpus);
        cpus_read_unlock();
        return ret;
}
EXPORT_SYMBOL_GPL(stop_machine);

/**
 * stop_machine_from_inactive_cpu - stop_machine() from inactive CPU
 * @fn: the function to run
 * @data: the data ptr for the @fn()
 * @cpus: the cpus to run the @fn() on (NULL = any online cpu)
 *
 * This is identical to stop_machine() but can be called from a CPU which
 * is not active.  The local CPU is in the process of hotplug (so no other
 * CPU hotplug can start) and not marked active and doesn't have enough
 * context to sleep.
 *
 * This function provides stop_machine() functionality for such state by
 * using busy-wait for synchronization and executing @fn directly for local
 * CPU.
 *
 * CONTEXT:
 * Local CPU is inactive.  Temporarily stops all active CPUs.
 *
 * RETURNS:
 * 0 if all executions of @fn returned 0, any non zero return value if any
 * returned non zero.
 */
int stop_machine_from_inactive_cpu(cpu_stop_fn_t fn, void *data,
                                  const struct cpumask *cpus)
{
        struct multi_stop_data msdata = { .fn = fn, .data = data,
                                            .active_cpus = cpus };
        struct cpu_stop_done done;
        int ret;

        /* Local CPU must be inactive and CPU hotplug in progress. */
        BUG_ON(cpu_active(raw_smp_processor_id()));
        msdata.num_threads = num_active_cpus() + 1;     /* +1 for local */

        /* No proper task established and can't sleep - busy wait for lock. */
        while (!mutex_trylock(&stop_cpus_mutex))
                cpu_relax();

        /* Schedule work on other CPUs and execute directly for local CPU */
        set_state(&msdata, MULTI_STOP_PREPARE);
        cpu_stop_init_done(&done, num_active_cpus());
        queue_stop_cpus_work(cpu_active_mask, multi_cpu_stop, &msdata,
                             &done);
        ret = multi_cpu_stop(&msdata);

        /* Busy wait for completion. */
        while (!completion_done(&done.completion))
                cpu_relax();

        mutex_unlock(&stop_cpus_mutex);
        return ret ?: done.ret;
}