/*
   * Generic helpers for smp ipi calls
   *
   * (C) Jens Axboe <jens.axboe@oracle.com> 2008

   */

  
  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

  #include <linux/irq_work.h>

  #include <linux/rcupdate.h>

  #include <linux/rculist.h>

  #include <linux/kernel.h>

  #include <linux/export.h>

  #include <linux/percpu.h>
  #include <linux/init.h>

  #include <linux/gfp.h>

  #include <linux/smp.h>

  #include <linux/cpu.h>

  #include <linux/sched.h>

  #include <linux/sched/idle.h>

  #include <linux/hypervisor.h>

  #include "smpboot.h"

  enum {

  	CSD_FLAG_LOCK		= 0x01,

  	CSD_FLAG_SYNCHRONOUS	= 0x02,

  };
  
  struct call_function_data {

  	call_single_data_t	__percpu *csd;

  	cpumask_var_t		cpumask;

  	cpumask_var_t		cpumask_ipi;

  };

  static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_function_data, cfd_data);

  static DEFINE_PER_CPU_SHARED_ALIGNED(struct llist_head, call_single_queue);

  static void flush_smp_call_function_queue(bool warn_cpu_offline);

  int smpcfd_prepare_cpu(unsigned int cpu)

  {

  	struct call_function_data *cfd = &per_cpu(cfd_data, cpu);

  	if (!zalloc_cpumask_var_node(&cfd->cpumask, GFP_KERNEL,
  				     cpu_to_node(cpu)))
  		return -ENOMEM;

  	if (!zalloc_cpumask_var_node(&cfd->cpumask_ipi, GFP_KERNEL,
  				     cpu_to_node(cpu))) {
  		free_cpumask_var(cfd->cpumask);
  		return -ENOMEM;
  	}

  	cfd->csd = alloc_percpu(call_single_data_t);

  	if (!cfd->csd) {

  		free_cpumask_var(cfd->cpumask);

  		free_cpumask_var(cfd->cpumask_ipi);

  		return -ENOMEM;
  	}
  
  	return 0;

  }

  int smpcfd_dead_cpu(unsigned int cpu)
  {
  	struct call_function_data *cfd = &per_cpu(cfd_data, cpu);
  
  	free_cpumask_var(cfd->cpumask);

  	free_cpumask_var(cfd->cpumask_ipi);

  	free_percpu(cfd->csd);
  	return 0;
  }
  
  int smpcfd_dying_cpu(unsigned int cpu)
  {
  	/*
  	 * The IPIs for the smp-call-function callbacks queued by other
  	 * CPUs might arrive late, either due to hardware latencies or
  	 * because this CPU disabled interrupts (inside stop-machine)
  	 * before the IPIs were sent. So flush out any pending callbacks
  	 * explicitly (without waiting for the IPIs to arrive), to
  	 * ensure that the outgoing CPU doesn't go offline with work
  	 * still pending.
  	 */
  	flush_smp_call_function_queue(false);
  	return 0;
  }

  void __init call_function_init(void)

  {
  	int i;

  	for_each_possible_cpu(i)
  		init_llist_head(&per_cpu(call_single_queue, i));

  	smpcfd_prepare_cpu(smp_processor_id());

  }

  /*

   * csd_lock/csd_unlock used to serialize access to per-cpu csd resources
   *

   * For non-synchronous ipi calls the csd can still be in use by the
   * previous function call. For multi-cpu calls its even more interesting
   * as we'll have to ensure no other cpu is observing our csd.

   */

  static __always_inline void csd_lock_wait(call_single_data_t *csd)

  {

  	smp_cond_load_acquire(&csd->flags, !(VAL & CSD_FLAG_LOCK));

  }

  static __always_inline void csd_lock(call_single_data_t *csd)

  {

  	csd_lock_wait(csd);
  	csd->flags |= CSD_FLAG_LOCK;

  
  	/*

  	 * prevent CPU from reordering the above assignment
  	 * to ->flags with any subsequent assignments to other

  	 * fields of the specified call_single_data_t structure:

  	 */

  	smp_wmb();

  }

  static __always_inline void csd_unlock(call_single_data_t *csd)

  {

  	WARN_ON(!(csd->flags & CSD_FLAG_LOCK));

  	/*

  	 * ensure we're all done before releasing data:

  	 */

  	smp_store_release(&csd->flags, 0);

  }

  static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);

  /*

   * Insert a previously allocated call_single_data_t element

   * for execution on the given CPU. data must already have
   * ->func, ->info, and ->flags set.

   */

  static int generic_exec_single(int cpu, call_single_data_t *csd,

  			       smp_call_func_t func, void *info)

  {

  	if (cpu == smp_processor_id()) {

  		unsigned long flags;
  
  		/*
  		 * We can unlock early even for the synchronous on-stack case,
  		 * since we're doing this from the same CPU..
  		 */
  		csd_unlock(csd);

  		local_irq_save(flags);
  		func(info);
  		local_irq_restore(flags);
  		return 0;
  	}

  	if ((unsigned)cpu >= nr_cpu_ids || !cpu_online(cpu)) {
  		csd_unlock(csd);

  		return -ENXIO;

  	}

  	csd->func = func;
  	csd->info = info;

  	/*

  	 * The list addition should be visible before sending the IPI
  	 * handler locks the list to pull the entry off it because of
  	 * normal cache coherency rules implied by spinlocks.
  	 *
  	 * If IPIs can go out of order to the cache coherency protocol
  	 * in an architecture, sufficient synchronisation should be added
  	 * to arch code to make it appear to obey cache coherency WRT

  	 * locking and barrier primitives. Generic code isn't really
  	 * equipped to do the right thing...

  	 */

  	if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu)))

  		arch_send_call_function_single_ipi(cpu);

  	return 0;

  }

  /**
   * generic_smp_call_function_single_interrupt - Execute SMP IPI callbacks
   *
   * Invoked by arch to handle an IPI for call function single.
   * Must be called with interrupts disabled.

   */
  void generic_smp_call_function_single_interrupt(void)
  {

  	flush_smp_call_function_queue(true);
  }
  
  /**
   * flush_smp_call_function_queue - Flush pending smp-call-function callbacks
   *
   * @warn_cpu_offline: If set to 'true', warn if callbacks were queued on an
   *		      offline CPU. Skip this check if set to 'false'.
   *
   * Flush any pending smp-call-function callbacks queued on this CPU. This is
   * invoked by the generic IPI handler, as well as by a CPU about to go offline,
   * to ensure that all pending IPI callbacks are run before it goes completely
   * offline.
   *
   * Loop through the call_single_queue and run all the queued callbacks.
   * Must be called with interrupts disabled.
   */
  static void flush_smp_call_function_queue(bool warn_cpu_offline)
  {
  	struct llist_head *head;

  	struct llist_node *entry;

  	call_single_data_t *csd, *csd_next;

  	static bool warned;

  	lockdep_assert_irqs_disabled();

  	head = this_cpu_ptr(&call_single_queue);

  	entry = llist_del_all(head);

  	entry = llist_reverse_order(entry);

  	/* There shouldn't be any pending callbacks on an offline CPU. */
  	if (unlikely(warn_cpu_offline && !cpu_online(smp_processor_id()) &&
  		     !warned && !llist_empty(head))) {

  		warned = true;
  		WARN(1, "IPI on offline CPU %d
  ", smp_processor_id());
  
  		/*
  		 * We don't have to use the _safe() variant here
  		 * because we are not invoking the IPI handlers yet.
  		 */
  		llist_for_each_entry(csd, entry, llist)
  			pr_warn("IPI callback %pS sent to offline CPU
  ",
  				csd->func);
  	}

  	llist_for_each_entry_safe(csd, csd_next, entry, llist) {

  		smp_call_func_t func = csd->func;
  		void *info = csd->info;
  
  		/* Do we wait until *after* callback? */
  		if (csd->flags & CSD_FLAG_SYNCHRONOUS) {
  			func(info);
  			csd_unlock(csd);
  		} else {
  			csd_unlock(csd);
  			func(info);
  		}

  	}

  
  	/*
  	 * Handle irq works queued remotely by irq_work_queue_on().
  	 * Smp functions above are typically synchronous so they
  	 * better run first since some other CPUs may be busy waiting
  	 * for them.
  	 */
  	irq_work_run();

  }
  
  /*
   * smp_call_function_single - Run a function on a specific CPU
   * @func: The function to run. This must be fast and non-blocking.
   * @info: An arbitrary pointer to pass to the function.

   * @wait: If true, wait until function has completed on other CPUs.
   *

   * Returns 0 on success, else a negative status code.

   */

  int smp_call_function_single(int cpu, smp_call_func_t func, void *info,

  			     int wait)

  {

  	call_single_data_t *csd;
  	call_single_data_t csd_stack = {
  		.flags = CSD_FLAG_LOCK | CSD_FLAG_SYNCHRONOUS,
  	};

  	int this_cpu;

  	int err;

  	/*
  	 * prevent preemption and reschedule on another processor,
  	 * as well as CPU removal
  	 */
  	this_cpu = get_cpu();

  	/*
  	 * Can deadlock when called with interrupts disabled.
  	 * We allow cpu's that are not yet online though, as no one else can
  	 * send smp call function interrupt to this cpu and as such deadlocks
  	 * can't happen.
  	 */
  	WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
  		     && !oops_in_progress);

  	csd = &csd_stack;
  	if (!wait) {
  		csd = this_cpu_ptr(&csd_data);
  		csd_lock(csd);
  	}
  
  	err = generic_exec_single(cpu, csd, func, info);
  
  	if (wait)
  		csd_lock_wait(csd);

  
  	put_cpu();

  	return err;

  }
  EXPORT_SYMBOL(smp_call_function_single);

  /**

   * smp_call_function_single_async(): Run an asynchronous function on a
   * 			         specific CPU.

   * @cpu: The CPU to run on.
   * @csd: Pre-allocated and setup data structure

   *

   * Like smp_call_function_single(), but the call is asynchonous and
   * can thus be done from contexts with disabled interrupts.
   *
   * The caller passes his own pre-allocated data structure
   * (ie: embedded in an object) and is responsible for synchronizing it
   * such that the IPIs performed on the @csd are strictly serialized.
   *
   * NOTE: Be careful, there is unfortunately no current debugging facility to
   * validate the correctness of this serialization.

   */

  int smp_call_function_single_async(int cpu, call_single_data_t *csd)

  {
  	int err = 0;

  	preempt_disable();

  
  	/* We could deadlock if we have to wait here with interrupts disabled! */
  	if (WARN_ON_ONCE(csd->flags & CSD_FLAG_LOCK))
  		csd_lock_wait(csd);
  
  	csd->flags = CSD_FLAG_LOCK;
  	smp_wmb();
  
  	err = generic_exec_single(cpu, csd, csd->func, csd->info);

  	preempt_enable();

  
  	return err;
  }

  EXPORT_SYMBOL_GPL(smp_call_function_single_async);

  /*
   * smp_call_function_any - Run a function on any of the given cpus
   * @mask: The mask of cpus it can run on.
   * @func: The function to run. This must be fast and non-blocking.
   * @info: An arbitrary pointer to pass to the function.
   * @wait: If true, wait until function has completed.
   *
   * Returns 0 on success, else a negative status code (if no cpus were online).

   *
   * Selection preference:
   *	1) current cpu if in @mask
   *	2) any cpu of current node if in @mask
   *	3) any other online cpu in @mask
   */
  int smp_call_function_any(const struct cpumask *mask,

  			  smp_call_func_t func, void *info, int wait)

  {
  	unsigned int cpu;
  	const struct cpumask *nodemask;
  	int ret;
  
  	/* Try for same CPU (cheapest) */
  	cpu = get_cpu();
  	if (cpumask_test_cpu(cpu, mask))
  		goto call;
  
  	/* Try for same node. */

  	nodemask = cpumask_of_node(cpu_to_node(cpu));

  	for (cpu = cpumask_first_and(nodemask, mask); cpu < nr_cpu_ids;
  	     cpu = cpumask_next_and(cpu, nodemask, mask)) {
  		if (cpu_online(cpu))
  			goto call;
  	}
  
  	/* Any online will do: smp_call_function_single handles nr_cpu_ids. */
  	cpu = cpumask_any_and(mask, cpu_online_mask);
  call:
  	ret = smp_call_function_single(cpu, func, info, wait);
  	put_cpu();
  	return ret;
  }
  EXPORT_SYMBOL_GPL(smp_call_function_any);

  /**

   * smp_call_function_many(): Run a function on a set of other CPUs.
   * @mask: The set of cpus to run on (only runs on online subset).

   * @func: The function to run. This must be fast and non-blocking.
   * @info: An arbitrary pointer to pass to the function.

   * @wait: If true, wait (atomically) until function has completed
   *        on other CPUs.

   *

   * If @wait is true, then returns once @func has returned.

   *
   * You must not call this function with disabled interrupts or from a
   * hardware interrupt handler or from a bottom half handler. Preemption
   * must be disabled when calling this function.
   */

  void smp_call_function_many(const struct cpumask *mask,

  			    smp_call_func_t func, void *info, bool wait)

  {

  	struct call_function_data *cfd;

  	int cpu, next_cpu, this_cpu = smp_processor_id();

  	/*
  	 * Can deadlock when called with interrupts disabled.
  	 * We allow cpu's that are not yet online though, as no one else can
  	 * send smp call function interrupt to this cpu and as such deadlocks
  	 * can't happen.
  	 */
  	WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()

  		     && !oops_in_progress && !early_boot_irqs_disabled);

  	/* Try to fastpath.  So, what's a CPU they want? Ignoring this one. */

  	cpu = cpumask_first_and(mask, cpu_online_mask);

  	if (cpu == this_cpu)

  		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);

  	/* No online cpus?  We're done. */
  	if (cpu >= nr_cpu_ids)
  		return;
  
  	/* Do we have another CPU which isn't us? */
  	next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);

  	if (next_cpu == this_cpu)

  		next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
  
  	/* Fastpath: do that cpu by itself. */
  	if (next_cpu >= nr_cpu_ids) {
  		smp_call_function_single(cpu, func, info, wait);
  		return;

  	}

  	cfd = this_cpu_ptr(&cfd_data);

  	cpumask_and(cfd->cpumask, mask, cpu_online_mask);

  	__cpumask_clear_cpu(this_cpu, cfd->cpumask);

  
  	/* Some callers race with other cpus changing the passed mask */

  	if (unlikely(!cpumask_weight(cfd->cpumask)))

  		return;

  	cpumask_clear(cfd->cpumask_ipi);

  	for_each_cpu(cpu, cfd->cpumask) {

  		call_single_data_t *csd = per_cpu_ptr(cfd->csd, cpu);

  
  		csd_lock(csd);

  		if (wait)
  			csd->flags |= CSD_FLAG_SYNCHRONOUS;

  		csd->func = func;
  		csd->info = info;

  		if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu)))

  			__cpumask_set_cpu(cpu, cfd->cpumask_ipi);

  	}

  	/* Send a message to all CPUs in the map */

  	arch_send_call_function_ipi_mask(cfd->cpumask_ipi);

  	if (wait) {

  		for_each_cpu(cpu, cfd->cpumask) {

  			call_single_data_t *csd;

  
  			csd = per_cpu_ptr(cfd->csd, cpu);

  			csd_lock_wait(csd);
  		}
  	}

  }

  EXPORT_SYMBOL(smp_call_function_many);

  
  /**
   * smp_call_function(): Run a function on all other CPUs.
   * @func: The function to run. This must be fast and non-blocking.
   * @info: An arbitrary pointer to pass to the function.

   * @wait: If true, wait (atomically) until function has completed
   *        on other CPUs.

   *

   * Returns 0.

   *
   * If @wait is true, then returns once @func has returned; otherwise

   * it returns just before the target cpu calls @func.

   *
   * You must not call this function with disabled interrupts or from a
   * hardware interrupt handler or from a bottom half handler.
   */

  int smp_call_function(smp_call_func_t func, void *info, int wait)

  {

  	preempt_disable();

  	smp_call_function_many(cpu_online_mask, func, info, wait);

  	preempt_enable();

  	return 0;

  }
  EXPORT_SYMBOL(smp_call_function);

  /* Setup configured maximum number of CPUs to activate */
  unsigned int setup_max_cpus = NR_CPUS;
  EXPORT_SYMBOL(setup_max_cpus);
  
  
  /*
   * Setup routine for controlling SMP activation
   *
   * Command-line option of "nosmp" or "maxcpus=0" will disable SMP
   * activation entirely (the MPS table probe still happens, though).
   *
   * Command-line option of "maxcpus=<NUM>", where <NUM> is an integer
   * greater than 0, limits the maximum number of CPUs activated in
   * SMP mode to <NUM>.
   */
  
  void __weak arch_disable_smp_support(void) { }
  
  static int __init nosmp(char *str)
  {
  	setup_max_cpus = 0;
  	arch_disable_smp_support();
  
  	return 0;
  }
  
  early_param("nosmp", nosmp);
  
  /* this is hard limit */
  static int __init nrcpus(char *str)
  {
  	int nr_cpus;
  
  	get_option(&str, &nr_cpus);
  	if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
  		nr_cpu_ids = nr_cpus;
  
  	return 0;
  }
  
  early_param("nr_cpus", nrcpus);
  
  static int __init maxcpus(char *str)
  {
  	get_option(&str, &setup_max_cpus);
  	if (setup_max_cpus == 0)
  		arch_disable_smp_support();
  
  	return 0;
  }
  
  early_param("maxcpus", maxcpus);
  
  /* Setup number of possible processor ids */

  unsigned int nr_cpu_ids __read_mostly = NR_CPUS;

  EXPORT_SYMBOL(nr_cpu_ids);
  
  /* An arch may set nr_cpu_ids earlier if needed, so this would be redundant */
  void __init setup_nr_cpu_ids(void)
  {
  	nr_cpu_ids = find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1;
  }
  
  /* Called by boot processor to activate the rest. */
  void __init smp_init(void)
  {

  	int num_nodes, num_cpus;

  	unsigned int cpu;

  	idle_threads_init();

  	cpuhp_threads_init();

  	pr_info("Bringing up secondary CPUs ...
  ");

  	/* FIXME: This should be done in userspace --RR */
  	for_each_present_cpu(cpu) {
  		if (num_online_cpus() >= setup_max_cpus)
  			break;
  		if (!cpu_online(cpu))
  			cpu_up(cpu);
  	}

  	num_nodes = num_online_nodes();
  	num_cpus  = num_online_cpus();
  	pr_info("Brought up %d node%s, %d CPU%s
  ",
  		num_nodes, (num_nodes > 1 ? "s" : ""),
  		num_cpus,  (num_cpus  > 1 ? "s" : ""));

  	/* Any cleanup work */

  	smp_cpus_done(setup_max_cpus);
  }

  /*

   * Call a function on all processors.  May be used during early boot while
   * early_boot_irqs_disabled is set.  Use local_irq_save/restore() instead
   * of local_irq_disable/enable().

   */
  int on_each_cpu(void (*func) (void *info), void *info, int wait)
  {

  	unsigned long flags;

  	int ret = 0;
  
  	preempt_disable();
  	ret = smp_call_function(func, info, wait);

  	local_irq_save(flags);

  	func(info);

  	local_irq_restore(flags);

  	preempt_enable();
  	return ret;
  }
  EXPORT_SYMBOL(on_each_cpu);

  
  /**
   * on_each_cpu_mask(): Run a function on processors specified by
   * cpumask, which may include the local processor.
   * @mask: The set of cpus to run on (only runs on online subset).
   * @func: The function to run. This must be fast and non-blocking.
   * @info: An arbitrary pointer to pass to the function.
   * @wait: If true, wait (atomically) until function has completed
   *        on other CPUs.
   *
   * If @wait is true, then returns once @func has returned.
   *

   * You must not call this function with disabled interrupts or from a
   * hardware interrupt handler or from a bottom half handler.  The
   * exception is that it may be used during early boot while
   * early_boot_irqs_disabled is set.

   */
  void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
  			void *info, bool wait)
  {
  	int cpu = get_cpu();
  
  	smp_call_function_many(mask, func, info, wait);
  	if (cpumask_test_cpu(cpu, mask)) {

  		unsigned long flags;
  		local_irq_save(flags);

  		func(info);

  		local_irq_restore(flags);

  	}
  	put_cpu();
  }
  EXPORT_SYMBOL(on_each_cpu_mask);

  
  /*
   * on_each_cpu_cond(): Call a function on each processor for which
   * the supplied function cond_func returns true, optionally waiting
   * for all the required CPUs to finish. This may include the local
   * processor.
   * @cond_func:	A callback function that is passed a cpu id and
   *		the the info parameter. The function is called
   *		with preemption disabled. The function should
   *		return a blooean value indicating whether to IPI
   *		the specified CPU.
   * @func:	The function to run on all applicable CPUs.
   *		This must be fast and non-blocking.
   * @info:	An arbitrary pointer to pass to both functions.
   * @wait:	If true, wait (atomically) until function has
   *		completed on other CPUs.
   * @gfp_flags:	GFP flags to use when allocating the cpumask
   *		used internally by the function.
   *
   * The function might sleep if the GFP flags indicates a non
   * atomic allocation is allowed.
   *
   * Preemption is disabled to protect against CPUs going offline but not online.
   * CPUs going online during the call will not be seen or sent an IPI.
   *
   * You must not call this function with disabled interrupts or
   * from a hardware interrupt handler or from a bottom half handler.
   */
  void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info),
  			smp_call_func_t func, void *info, bool wait,
  			gfp_t gfp_flags)
  {
  	cpumask_var_t cpus;
  	int cpu, ret;

  	might_sleep_if(gfpflags_allow_blocking(gfp_flags));

  
  	if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) {
  		preempt_disable();
  		for_each_online_cpu(cpu)
  			if (cond_func(cpu, info))
  				cpumask_set_cpu(cpu, cpus);
  		on_each_cpu_mask(cpus, func, info, wait);
  		preempt_enable();
  		free_cpumask_var(cpus);
  	} else {
  		/*
  		 * No free cpumask, bother. No matter, we'll
  		 * just have to IPI them one by one.
  		 */
  		preempt_disable();
  		for_each_online_cpu(cpu)
  			if (cond_func(cpu, info)) {
  				ret = smp_call_function_single(cpu, func,
  								info, wait);

  				WARN_ON_ONCE(ret);

  			}
  		preempt_enable();
  	}
  }
  EXPORT_SYMBOL(on_each_cpu_cond);

  
  static void do_nothing(void *unused)
  {
  }
  
  /**
   * kick_all_cpus_sync - Force all cpus out of idle
   *
   * Used to synchronize the update of pm_idle function pointer. It's
   * called after the pointer is updated and returns after the dummy
   * callback function has been executed on all cpus. The execution of
   * the function can only happen on the remote cpus after they have
   * left the idle function which had been called via pm_idle function
   * pointer. So it's guaranteed that nothing uses the previous pointer
   * anymore.
   */
  void kick_all_cpus_sync(void)
  {
  	/* Make sure the change is visible before we kick the cpus */
  	smp_mb();
  	smp_call_function(do_nothing, NULL, 1);
  }
  EXPORT_SYMBOL_GPL(kick_all_cpus_sync);

  
  /**
   * wake_up_all_idle_cpus - break all cpus out of idle
   * wake_up_all_idle_cpus try to break all cpus which is in idle state even
   * including idle polling cpus, for non-idle cpus, we will do nothing
   * for them.
   */
  void wake_up_all_idle_cpus(void)
  {
  	int cpu;
  
  	preempt_disable();
  	for_each_online_cpu(cpu) {
  		if (cpu == smp_processor_id())
  			continue;
  
  		wake_up_if_idle(cpu);
  	}
  	preempt_enable();
  }
  EXPORT_SYMBOL_GPL(wake_up_all_idle_cpus);

  
  /**
   * smp_call_on_cpu - Call a function on a specific cpu
   *
   * Used to call a function on a specific cpu and wait for it to return.
   * Optionally make sure the call is done on a specified physical cpu via vcpu
   * pinning in order to support virtualized environments.
   */
  struct smp_call_on_cpu_struct {
  	struct work_struct	work;
  	struct completion	done;
  	int			(*func)(void *);
  	void			*data;
  	int			ret;
  	int			cpu;
  };
  
  static void smp_call_on_cpu_callback(struct work_struct *work)
  {
  	struct smp_call_on_cpu_struct *sscs;
  
  	sscs = container_of(work, struct smp_call_on_cpu_struct, work);
  	if (sscs->cpu >= 0)
  		hypervisor_pin_vcpu(sscs->cpu);
  	sscs->ret = sscs->func(sscs->data);
  	if (sscs->cpu >= 0)
  		hypervisor_pin_vcpu(-1);
  
  	complete(&sscs->done);
  }
  
  int smp_call_on_cpu(unsigned int cpu, int (*func)(void *), void *par, bool phys)
  {
  	struct smp_call_on_cpu_struct sscs = {

  		.done = COMPLETION_INITIALIZER_ONSTACK(sscs.done),
  		.func = func,
  		.data = par,
  		.cpu  = phys ? cpu : -1,
  	};

  	INIT_WORK_ONSTACK(&sscs.work, smp_call_on_cpu_callback);

  	if (cpu >= nr_cpu_ids || !cpu_online(cpu))
  		return -ENXIO;
  
  	queue_work_on(cpu, system_wq, &sscs.work);
  	wait_for_completion(&sscs.done);
  
  	return sscs.ret;
  }
  EXPORT_SYMBOL_GPL(smp_call_on_cpu);