#include <xen/percpu.h>
#include <xen/cpu.h>
#include <xen/init.h>
#include <xen/mm.h>
#include <xen/rcupdate.h>

unsigned long __per_cpu_offset[NR_CPUS];

/*
 * Force uses of per_cpu() with an invalid area to attempt to access the
 * middle of the non-canonical address space resulting in a #GP, rather than a
 * possible #PF at (NULL + a little) which has security implications in the
 * context of PV guests.
 */
#define INVALID_PERCPU_AREA (0x8000000000000000L - (long)__per_cpu_start)
#define PERCPU_ORDER (get_order_from_bytes(__per_cpu_data_end-__per_cpu_start))

void __init percpu_init_areas(void)
{
    unsigned int cpu;
    for ( cpu = 1; cpu < NR_CPUS; cpu++ )
        __per_cpu_offset[cpu] = INVALID_PERCPU_AREA;
}

static int init_percpu_area(unsigned int cpu)
{
    char *p;
    if ( __per_cpu_offset[cpu] != INVALID_PERCPU_AREA )
        return -EBUSY;
    if ( (p = alloc_xenheap_pages(PERCPU_ORDER, 0)) == NULL )
        return -ENOMEM;
    memset(p, 0, __per_cpu_data_end - __per_cpu_start);
    __per_cpu_offset[cpu] = p - __per_cpu_start;
    return 0;
}

struct free_info {
    unsigned int cpu;
    struct rcu_head rcu;
};
static DEFINE_PER_CPU(struct free_info, free_info);

static void _free_percpu_area(struct rcu_head *head)
{
    struct free_info *info = container_of(head, struct free_info, rcu);
    unsigned int cpu = info->cpu;
    char *p = __per_cpu_start + __per_cpu_offset[cpu];
    free_xenheap_pages(p, PERCPU_ORDER);
    __per_cpu_offset[cpu] = INVALID_PERCPU_AREA;
}

static void free_percpu_area(unsigned int cpu)
{
    struct free_info *info = &per_cpu(free_info, cpu);
    info->cpu = cpu;
    call_rcu(&info->rcu, _free_percpu_area);
}

static int cpu_percpu_callback(
    struct notifier_block *nfb, unsigned long action, void *hcpu)
{
    unsigned int cpu = (unsigned long)hcpu;
    int rc = 0;

    switch ( action )
    {
    case CPU_UP_PREPARE:
        rc = init_percpu_area(cpu);
        break;
    case CPU_UP_CANCELED:
    case CPU_DEAD:
        free_percpu_area(cpu);
        break;
    default:
        break;
    }

    return !rc ? NOTIFY_DONE : notifier_from_errno(rc);
}

static struct notifier_block cpu_percpu_nfb = {
    .notifier_call = cpu_percpu_callback,
    .priority = 100 /* highest priority */
};

static int __init percpu_presmp_init(void)
{
    register_cpu_notifier(&cpu_percpu_nfb);
    return 0;
}
presmp_initcall(percpu_presmp_init);
