[PATCH] x86: Save the MTRRs of the BSP before booting an AP

[deliverable/linux.git] / arch / x86_64 / kernel / smpboot.c

/*
 *      x86 SMP booting functions
 *
 *      (c) 1995 Alan Cox, Building #3 <alan@redhat.com>
 *      (c) 1998, 1999, 2000 Ingo Molnar <mingo@redhat.com>
 *      Copyright 2001 Andi Kleen, SuSE Labs.
 *
 *      Much of the core SMP work is based on previous work by Thomas Radke, to
 *      whom a great many thanks are extended.
 *
 *      Thanks to Intel for making available several different Pentium,
 *      Pentium Pro and Pentium-II/Xeon MP machines.
 *      Original development of Linux SMP code supported by Caldera.
 *
 *      This code is released under the GNU General Public License version 2
 *
 *      Fixes
 *              Felix Koop      :       NR_CPUS used properly
 *              Jose Renau      :       Handle single CPU case.
 *              Alan Cox        :       By repeated request 8) - Total BogoMIP report.
 *              Greg Wright     :       Fix for kernel stacks panic.
 *              Erich Boleyn    :       MP v1.4 and additional changes.
 *      Matthias Sattler        :       Changes for 2.1 kernel map.
 *      Michel Lespinasse       :       Changes for 2.1 kernel map.
 *      Michael Chastain        :       Change trampoline.S to gnu as.
 *              Alan Cox        :       Dumb bug: 'B' step PPro's are fine
 *              Ingo Molnar     :       Added APIC timers, based on code
 *                                      from Jose Renau
 *              Ingo Molnar     :       various cleanups and rewrites
 *              Tigran Aivazian :       fixed "0.00 in /proc/uptime on SMP" bug.
 *      Maciej W. Rozycki       :       Bits for genuine 82489DX APICs
 *      Andi Kleen              :       Changed for SMP boot into long mode.
 *              Rusty Russell   :       Hacked into shape for new "hotplug" boot process.
 *      Andi Kleen              :       Converted to new state machine.
 *                                      Various cleanups.
 *                                      Probably mostly hotplug CPU ready now.
 *      Ashok Raj                       : CPU hotplug support
 */


#include <linux/init.h>

#include <linux/mm.h>
#include <linux/kernel_stat.h>
#include <linux/smp_lock.h>
#include <linux/bootmem.h>
#include <linux/thread_info.h>
#include <linux/module.h>
#include <linux/delay.h>
#include <linux/mc146818rtc.h>
#include <linux/smp.h>

#include <asm/mtrr.h>
#include <asm/pgalloc.h>
#include <asm/desc.h>
#include <asm/kdebug.h>
#include <asm/tlbflush.h>
#include <asm/proto.h>
#include <asm/nmi.h>
#include <asm/irq.h>
#include <asm/hw_irq.h>
#include <asm/numa.h>

/* Number of siblings per CPU package */
int smp_num_siblings = 1;
EXPORT_SYMBOL(smp_num_siblings);

/* Last level cache ID of each logical CPU */
u8 cpu_llc_id[NR_CPUS] __cpuinitdata  = {[0 ... NR_CPUS-1] = BAD_APICID};
EXPORT_SYMBOL(cpu_llc_id);

/* Bitmask of currently online CPUs */
cpumask_t cpu_online_map __read_mostly;

EXPORT_SYMBOL(cpu_online_map);

/*
 * Private maps to synchronize booting between AP and BP.
 * Probably not needed anymore, but it makes for easier debugging. -AK
 */
cpumask_t cpu_callin_map;
cpumask_t cpu_callout_map;
EXPORT_SYMBOL(cpu_callout_map);

cpumask_t cpu_possible_map;
EXPORT_SYMBOL(cpu_possible_map);

/* Per CPU bogomips and other parameters */
struct cpuinfo_x86 cpu_data[NR_CPUS] __cacheline_aligned;
EXPORT_SYMBOL(cpu_data);

/* Set when the idlers are all forked */
int smp_threads_ready;

/* representing HT siblings of each logical CPU */
cpumask_t cpu_sibling_map[NR_CPUS] __read_mostly;
EXPORT_SYMBOL(cpu_sibling_map);

/* representing HT and core siblings of each logical CPU */
cpumask_t cpu_core_map[NR_CPUS] __read_mostly;
EXPORT_SYMBOL(cpu_core_map);

/*
 * Trampoline 80x86 program as an array.
 */

extern unsigned char trampoline_data[];
extern unsigned char trampoline_end[];

/* State of each CPU */
DEFINE_PER_CPU(int, cpu_state) = { 0 };

/*
 * Store all idle threads, this can be reused instead of creating
 * a new thread. Also avoids complicated thread destroy functionality
 * for idle threads.
 */
struct task_struct *idle_thread_array[NR_CPUS] __cpuinitdata ;

#define get_idle_for_cpu(x)     (idle_thread_array[(x)])
#define set_idle_for_cpu(x,p)   (idle_thread_array[(x)] = (p))

/*
 * Currently trivial. Write the real->protected mode
 * bootstrap into the page concerned. The caller
 * has made sure it's suitably aligned.
 */

static unsigned long __cpuinit setup_trampoline(void)
{
        void *tramp = __va(SMP_TRAMPOLINE_BASE); 
        memcpy(tramp, trampoline_data, trampoline_end - trampoline_data);
        return virt_to_phys(tramp);
}

/*
 * The bootstrap kernel entry code has set these up. Save them for
 * a given CPU
 */

static void __cpuinit smp_store_cpu_info(int id)
{
        struct cpuinfo_x86 *c = cpu_data + id;

        *c = boot_cpu_data;
        identify_cpu(c);
        print_cpu_info(c);
}

static atomic_t init_deasserted __cpuinitdata;

/*
 * Report back to the Boot Processor.
 * Running on AP.
 */
void __cpuinit smp_callin(void)
{
        int cpuid, phys_id;
        unsigned long timeout;

        /*
         * If waken up by an INIT in an 82489DX configuration
         * we may get here before an INIT-deassert IPI reaches
         * our local APIC.  We have to wait for the IPI or we'll
         * lock up on an APIC access.
         */
        while (!atomic_read(&init_deasserted))
                cpu_relax();

        /*
         * (This works even if the APIC is not enabled.)
         */
        phys_id = GET_APIC_ID(apic_read(APIC_ID));
        cpuid = smp_processor_id();
        if (cpu_isset(cpuid, cpu_callin_map)) {
                panic("smp_callin: phys CPU#%d, CPU#%d already present??\n",
                                        phys_id, cpuid);
        }
        Dprintk("CPU#%d (phys ID: %d) waiting for CALLOUT\n", cpuid, phys_id);

        /*
         * STARTUP IPIs are fragile beasts as they might sometimes
         * trigger some glue motherboard logic. Complete APIC bus
         * silence for 1 second, this overestimates the time the
         * boot CPU is spending to send the up to 2 STARTUP IPIs
         * by a factor of two. This should be enough.
         */

        /*
         * Waiting 2s total for startup (udelay is not yet working)
         */
        timeout = jiffies + 2*HZ;
        while (time_before(jiffies, timeout)) {
                /*
                 * Has the boot CPU finished it's STARTUP sequence?
                 */
                if (cpu_isset(cpuid, cpu_callout_map))
                        break;
                cpu_relax();
        }

        if (!time_before(jiffies, timeout)) {
                panic("smp_callin: CPU%d started up but did not get a callout!\n",
                        cpuid);
        }

        /*
         * the boot CPU has finished the init stage and is spinning
         * on callin_map until we finish. We are free to set up this
         * CPU, first the APIC. (this is probably redundant on most
         * boards)
         */

        Dprintk("CALLIN, before setup_local_APIC().\n");
        setup_local_APIC();

        /*
         * Get our bogomips.
         *
         * Need to enable IRQs because it can take longer and then
         * the NMI watchdog might kill us.
         */
        local_irq_enable();
        calibrate_delay();
        local_irq_disable();
        Dprintk("Stack at about %p\n",&cpuid);

        disable_APIC_timer();

        /*
         * Save our processor parameters
         */
        smp_store_cpu_info(cpuid);

        /*
         * Allow the master to continue.
         */
        cpu_set(cpuid, cpu_callin_map);
}

/* maps the cpu to the sched domain representing multi-core */
cpumask_t cpu_coregroup_map(int cpu)
{
        struct cpuinfo_x86 *c = cpu_data + cpu;
        /*
         * For perf, we return last level cache shared map.
         * And for power savings, we return cpu_core_map
         */
        if (sched_mc_power_savings || sched_smt_power_savings)
                return cpu_core_map[cpu];
        else
                return c->llc_shared_map;
}

/* representing cpus for which sibling maps can be computed */
static cpumask_t cpu_sibling_setup_map;

static inline void set_cpu_sibling_map(int cpu)
{
        int i;
        struct cpuinfo_x86 *c = cpu_data;

        cpu_set(cpu, cpu_sibling_setup_map);

        if (smp_num_siblings > 1) {
                for_each_cpu_mask(i, cpu_sibling_setup_map) {
                        if (c[cpu].phys_proc_id == c[i].phys_proc_id &&
                            c[cpu].cpu_core_id == c[i].cpu_core_id) {
                                cpu_set(i, cpu_sibling_map[cpu]);
                                cpu_set(cpu, cpu_sibling_map[i]);
                                cpu_set(i, cpu_core_map[cpu]);
                                cpu_set(cpu, cpu_core_map[i]);
                                cpu_set(i, c[cpu].llc_shared_map);
                                cpu_set(cpu, c[i].llc_shared_map);
                        }
                }
        } else {
                cpu_set(cpu, cpu_sibling_map[cpu]);
        }

        cpu_set(cpu, c[cpu].llc_shared_map);

        if (current_cpu_data.x86_max_cores == 1) {
                cpu_core_map[cpu] = cpu_sibling_map[cpu];
                c[cpu].booted_cores = 1;
                return;
        }

        for_each_cpu_mask(i, cpu_sibling_setup_map) {
                if (cpu_llc_id[cpu] != BAD_APICID &&
                    cpu_llc_id[cpu] == cpu_llc_id[i]) {
                        cpu_set(i, c[cpu].llc_shared_map);
                        cpu_set(cpu, c[i].llc_shared_map);
                }
                if (c[cpu].phys_proc_id == c[i].phys_proc_id) {
                        cpu_set(i, cpu_core_map[cpu]);
                        cpu_set(cpu, cpu_core_map[i]);
                        /*
                         *  Does this new cpu bringup a new core?
                         */
                        if (cpus_weight(cpu_sibling_map[cpu]) == 1) {
                                /*
                                 * for each core in package, increment
                                 * the booted_cores for this new cpu
                                 */
                                if (first_cpu(cpu_sibling_map[i]) == i)
                                        c[cpu].booted_cores++;
                                /*
                                 * increment the core count for all
                                 * the other cpus in this package
                                 */
                                if (i != cpu)
                                        c[i].booted_cores++;
                        } else if (i != cpu && !c[cpu].booted_cores)
                                c[cpu].booted_cores = c[i].booted_cores;
                }
        }
}

/*
 * Setup code on secondary processor (after comming out of the trampoline)
 */
void __cpuinit start_secondary(void)
{
        /*
         * Dont put anything before smp_callin(), SMP
         * booting is too fragile that we want to limit the
         * things done here to the most necessary things.
         */
        cpu_init();
        preempt_disable();
        smp_callin();

        /* otherwise gcc will move up the smp_processor_id before the cpu_init */
        barrier();

        /*
         * Check TSC sync first:
         */
        check_tsc_sync_target();

        Dprintk("cpu %d: setting up apic clock\n", smp_processor_id());         
        setup_secondary_APIC_clock();

        Dprintk("cpu %d: enabling apic timer\n", smp_processor_id());

        if (nmi_watchdog == NMI_IO_APIC) {
                disable_8259A_irq(0);
                enable_NMI_through_LVT0(NULL);
                enable_8259A_irq(0);
        }

        enable_APIC_timer();

        /*
         * The sibling maps must be set before turing the online map on for
         * this cpu
         */
        set_cpu_sibling_map(smp_processor_id());

        /*
         * We need to hold call_lock, so there is no inconsistency
         * between the time smp_call_function() determines number of
         * IPI receipients, and the time when the determination is made
         * for which cpus receive the IPI in genapic_flat.c. Holding this
         * lock helps us to not include this cpu in a currently in progress
         * smp_call_function().
         */
        lock_ipi_call_lock();
        spin_lock(&vector_lock);

        /* Setup the per cpu irq handling data structures */
        __setup_vector_irq(smp_processor_id());
        /*
         * Allow the master to continue.
         */
        cpu_set(smp_processor_id(), cpu_online_map);
        per_cpu(cpu_state, smp_processor_id()) = CPU_ONLINE;
        spin_unlock(&vector_lock);

        unlock_ipi_call_lock();

        cpu_idle();
}

extern volatile unsigned long init_rsp;
extern void (*initial_code)(void);

#ifdef APIC_DEBUG
static void inquire_remote_apic(int apicid)
{
        unsigned i, regs[] = { APIC_ID >> 4, APIC_LVR >> 4, APIC_SPIV >> 4 };
        char *names[] = { "ID", "VERSION", "SPIV" };
        int timeout, status;

        printk(KERN_INFO "Inquiring remote APIC #%d...\n", apicid);

        for (i = 0; i < sizeof(regs) / sizeof(*regs); i++) {
                printk("... APIC #%d %s: ", apicid, names[i]);

                /*
                 * Wait for idle.
                 */
                apic_wait_icr_idle();

                apic_write(APIC_ICR2, SET_APIC_DEST_FIELD(apicid));
                apic_write(APIC_ICR, APIC_DM_REMRD | regs[i]);

                timeout = 0;
                do {
                        udelay(100);
                        status = apic_read(APIC_ICR) & APIC_ICR_RR_MASK;
                } while (status == APIC_ICR_RR_INPROG && timeout++ < 1000);

                switch (status) {
                case APIC_ICR_RR_VALID:
                        status = apic_read(APIC_RRR);
                        printk("%08x\n", status);
                        break;
                default:
                        printk("failed\n");
                }
        }
}
#endif

/*
 * Kick the secondary to wake up.
 */
static int __cpuinit wakeup_secondary_via_INIT(int phys_apicid, unsigned int start_rip)
{
        unsigned long send_status = 0, accept_status = 0;
        int maxlvt, timeout, num_starts, j;

        Dprintk("Asserting INIT.\n");

        /*
         * Turn INIT on target chip
         */
        apic_write(APIC_ICR2, SET_APIC_DEST_FIELD(phys_apicid));

        /*
         * Send IPI
         */
        apic_write(APIC_ICR, APIC_INT_LEVELTRIG | APIC_INT_ASSERT
                                | APIC_DM_INIT);

        Dprintk("Waiting for send to finish...\n");
        timeout = 0;
        do {
                Dprintk("+");
                udelay(100);
                send_status = apic_read(APIC_ICR) & APIC_ICR_BUSY;
        } while (send_status && (timeout++ < 1000));

        mdelay(10);

        Dprintk("Deasserting INIT.\n");

        /* Target chip */
        apic_write(APIC_ICR2, SET_APIC_DEST_FIELD(phys_apicid));

        /* Send IPI */
        apic_write(APIC_ICR, APIC_INT_LEVELTRIG | APIC_DM_INIT);

        Dprintk("Waiting for send to finish...\n");
        timeout = 0;
        do {
                Dprintk("+");
                udelay(100);
                send_status = apic_read(APIC_ICR) & APIC_ICR_BUSY;
        } while (send_status && (timeout++ < 1000));

        mb();
        atomic_set(&init_deasserted, 1);

        num_starts = 2;

        /*
         * Run STARTUP IPI loop.
         */
        Dprintk("#startup loops: %d.\n", num_starts);

        maxlvt = get_maxlvt();

        for (j = 1; j <= num_starts; j++) {
                Dprintk("Sending STARTUP #%d.\n",j);
                apic_write(APIC_ESR, 0);
                apic_read(APIC_ESR);
                Dprintk("After apic_write.\n");

                /*
                 * STARTUP IPI
                 */

                /* Target chip */
                apic_write(APIC_ICR2, SET_APIC_DEST_FIELD(phys_apicid));

                /* Boot on the stack */
                /* Kick the second */
                apic_write(APIC_ICR, APIC_DM_STARTUP | (start_rip >> 12));

                /*
                 * Give the other CPU some time to accept the IPI.
                 */
                udelay(300);

                Dprintk("Startup point 1.\n");

                Dprintk("Waiting for send to finish...\n");
                timeout = 0;
                do {
                        Dprintk("+");
                        udelay(100);
                        send_status = apic_read(APIC_ICR) & APIC_ICR_BUSY;
                } while (send_status && (timeout++ < 1000));

                /*
                 * Give the other CPU some time to accept the IPI.
                 */
                udelay(200);
                /*
                 * Due to the Pentium erratum 3AP.
                 */
                if (maxlvt > 3) {
                        apic_write(APIC_ESR, 0);
                }
                accept_status = (apic_read(APIC_ESR) & 0xEF);
                if (send_status || accept_status)
                        break;
        }
        Dprintk("After Startup.\n");

        if (send_status)
                printk(KERN_ERR "APIC never delivered???\n");
        if (accept_status)
                printk(KERN_ERR "APIC delivery error (%lx).\n", accept_status);

        return (send_status | accept_status);
}

struct create_idle {
        struct work_struct work;
        struct task_struct *idle;
        struct completion done;
        int cpu;
};

void do_fork_idle(struct work_struct *work)
{
        struct create_idle *c_idle =
                container_of(work, struct create_idle, work);

        c_idle->idle = fork_idle(c_idle->cpu);
        complete(&c_idle->done);
}

/*
 * Boot one CPU.
 */
static int __cpuinit do_boot_cpu(int cpu, int apicid)
{
        unsigned long boot_error;
        int timeout;
        unsigned long start_rip;
        struct create_idle c_idle = {
                .work = __WORK_INITIALIZER(c_idle.work, do_fork_idle),
                .cpu = cpu,
                .done = COMPLETION_INITIALIZER_ONSTACK(c_idle.done),
        };

        /* allocate memory for gdts of secondary cpus. Hotplug is considered */
        if (!cpu_gdt_descr[cpu].address &&
                !(cpu_gdt_descr[cpu].address = get_zeroed_page(GFP_KERNEL))) {
                printk(KERN_ERR "Failed to allocate GDT for CPU %d\n", cpu);
                return -1;
        }

        /* Allocate node local memory for AP pdas */
        if (cpu_pda(cpu) == &boot_cpu_pda[cpu]) {
                struct x8664_pda *newpda, *pda;
                int node = cpu_to_node(cpu);
                pda = cpu_pda(cpu);
                newpda = kmalloc_node(sizeof (struct x8664_pda), GFP_ATOMIC,
                                      node);
                if (newpda) {
                        memcpy(newpda, pda, sizeof (struct x8664_pda));
                        cpu_pda(cpu) = newpda;
                } else
                        printk(KERN_ERR
                "Could not allocate node local PDA for CPU %d on node %d\n",
                                cpu, node);
        }

        alternatives_smp_switch(1);

        c_idle.idle = get_idle_for_cpu(cpu);

        if (c_idle.idle) {
                c_idle.idle->thread.rsp = (unsigned long) (((struct pt_regs *)
                        (THREAD_SIZE +  task_stack_page(c_idle.idle))) - 1);
                init_idle(c_idle.idle, cpu);
                goto do_rest;
        }

        /*
         * During cold boot process, keventd thread is not spun up yet.
         * When we do cpu hot-add, we create idle threads on the fly, we should
         * not acquire any attributes from the calling context. Hence the clean
         * way to create kernel_threads() is to do that from keventd().
         * We do the current_is_keventd() due to the fact that ACPI notifier
         * was also queuing to keventd() and when the caller is already running
         * in context of keventd(), we would end up with locking up the keventd
         * thread.
         */
        if (!keventd_up() || current_is_keventd())
                c_idle.work.func(&c_idle.work);
        else {
                schedule_work(&c_idle.work);
                wait_for_completion(&c_idle.done);
        }

        if (IS_ERR(c_idle.idle)) {
                printk("failed fork for CPU %d\n", cpu);
                return PTR_ERR(c_idle.idle);
        }

        set_idle_for_cpu(cpu, c_idle.idle);

do_rest:

        cpu_pda(cpu)->pcurrent = c_idle.idle;

        start_rip = setup_trampoline();

        init_rsp = c_idle.idle->thread.rsp;
        per_cpu(init_tss,cpu).rsp0 = init_rsp;
        initial_code = start_secondary;
        clear_tsk_thread_flag(c_idle.idle, TIF_FORK);

        printk(KERN_INFO "Booting processor %d/%d APIC 0x%x\n", cpu,
                cpus_weight(cpu_present_map),
                apicid);

        /*
         * This grunge runs the startup process for
         * the targeted processor.
         */

        atomic_set(&init_deasserted, 0);

        Dprintk("Setting warm reset code and vector.\n");

        CMOS_WRITE(0xa, 0xf);
        local_flush_tlb();
        Dprintk("1.\n");
        *((volatile unsigned short *) phys_to_virt(0x469)) = start_rip >> 4;
        Dprintk("2.\n");
        *((volatile unsigned short *) phys_to_virt(0x467)) = start_rip & 0xf;
        Dprintk("3.\n");

        /*
         * Be paranoid about clearing APIC errors.
         */
        apic_write(APIC_ESR, 0);
        apic_read(APIC_ESR);

        /*
         * Status is now clean
         */
        boot_error = 0;

        /*
         * Starting actual IPI sequence...
         */
        boot_error = wakeup_secondary_via_INIT(apicid, start_rip);

        if (!boot_error) {
                /*
                 * allow APs to start initializing.
                 */
                Dprintk("Before Callout %d.\n", cpu);
                cpu_set(cpu, cpu_callout_map);
                Dprintk("After Callout %d.\n", cpu);

                /*
                 * Wait 5s total for a response
                 */
                for (timeout = 0; timeout < 50000; timeout++) {
                        if (cpu_isset(cpu, cpu_callin_map))
                                break;  /* It has booted */
                        udelay(100);
                }

                if (cpu_isset(cpu, cpu_callin_map)) {
                        /* number CPUs logically, starting from 1 (BSP is 0) */
                        Dprintk("CPU has booted.\n");
                } else {
                        boot_error = 1;
                        if (*((volatile unsigned char *)phys_to_virt(SMP_TRAMPOLINE_BASE))
                                        == 0xA5)
                                /* trampoline started but...? */
                                printk("Stuck ??\n");
                        else
                                /* trampoline code not run */
                                printk("Not responding.\n");
#ifdef APIC_DEBUG
                        inquire_remote_apic(apicid);
#endif
                }
        }
        if (boot_error) {
                cpu_clear(cpu, cpu_callout_map); /* was set here (do_boot_cpu()) */
                clear_bit(cpu, &cpu_initialized); /* was set by cpu_init() */
                clear_node_cpumask(cpu); /* was set by numa_add_cpu */
                cpu_clear(cpu, cpu_present_map);
                cpu_clear(cpu, cpu_possible_map);
                x86_cpu_to_apicid[cpu] = BAD_APICID;
                x86_cpu_to_log_apicid[cpu] = BAD_APICID;
                return -EIO;
        }

        return 0;
}

cycles_t cacheflush_time;
unsigned long cache_decay_ticks;

/*
 * Cleanup possible dangling ends...
 */
static __cpuinit void smp_cleanup_boot(void)
{
        /*
         * Paranoid:  Set warm reset code and vector here back
         * to default values.
         */
        CMOS_WRITE(0, 0xf);

        /*
         * Reset trampoline flag
         */
        *((volatile int *) phys_to_virt(0x467)) = 0;
}

/*
 * Fall back to non SMP mode after errors.
 *
 * RED-PEN audit/test this more. I bet there is more state messed up here.
 */
static __init void disable_smp(void)
{
        cpu_present_map = cpumask_of_cpu(0);
        cpu_possible_map = cpumask_of_cpu(0);
        if (smp_found_config)
                phys_cpu_present_map = physid_mask_of_physid(boot_cpu_id);
        else
                phys_cpu_present_map = physid_mask_of_physid(0);
        cpu_set(0, cpu_sibling_map[0]);
        cpu_set(0, cpu_core_map[0]);
}

#ifdef CONFIG_HOTPLUG_CPU

int additional_cpus __initdata = -1;

/*
 * cpu_possible_map should be static, it cannot change as cpu's
 * are onlined, or offlined. The reason is per-cpu data-structures
 * are allocated by some modules at init time, and dont expect to
 * do this dynamically on cpu arrival/departure.
 * cpu_present_map on the other hand can change dynamically.
 * In case when cpu_hotplug is not compiled, then we resort to current
 * behaviour, which is cpu_possible == cpu_present.
 * - Ashok Raj
 *
 * Three ways to find out the number of additional hotplug CPUs:
 * - If the BIOS specified disabled CPUs in ACPI/mptables use that.
 * - The user can overwrite it with additional_cpus=NUM
 * - Otherwise don't reserve additional CPUs.
 * We do this because additional CPUs waste a lot of memory.
 * -AK
 */
__init void prefill_possible_map(void)
{
        int i;
        int possible;

        if (additional_cpus == -1) {
                if (disabled_cpus > 0)
                        additional_cpus = disabled_cpus;
                else
                        additional_cpus = 0;
        }
        possible = num_processors + additional_cpus;
        if (possible > NR_CPUS) 
                possible = NR_CPUS;

        printk(KERN_INFO "SMP: Allowing %d CPUs, %d hotplug CPUs\n",
                possible,
                max_t(int, possible - num_processors, 0));

        for (i = 0; i < possible; i++)
                cpu_set(i, cpu_possible_map);
}
#endif

/*
 * Various sanity checks.
 */
static int __init smp_sanity_check(unsigned max_cpus)
{
        if (!physid_isset(hard_smp_processor_id(), phys_cpu_present_map)) {
                printk("weird, boot CPU (#%d) not listed by the BIOS.\n",
                       hard_smp_processor_id());
                physid_set(hard_smp_processor_id(), phys_cpu_present_map);
        }

        /*
         * If we couldn't find an SMP configuration at boot time,
         * get out of here now!
         */
        if (!smp_found_config) {
                printk(KERN_NOTICE "SMP motherboard not detected.\n");
                disable_smp();
                if (APIC_init_uniprocessor())
                        printk(KERN_NOTICE "Local APIC not detected."
                                           " Using dummy APIC emulation.\n");
                return -1;
        }

        /*
         * Should not be necessary because the MP table should list the boot
         * CPU too, but we do it for the sake of robustness anyway.
         */
        if (!physid_isset(boot_cpu_id, phys_cpu_present_map)) {
                printk(KERN_NOTICE "weird, boot CPU (#%d) not listed by the BIOS.\n",
                                                                 boot_cpu_id);
                physid_set(hard_smp_processor_id(), phys_cpu_present_map);
        }

        /*
         * If we couldn't find a local APIC, then get out of here now!
         */
        if (!cpu_has_apic) {
                printk(KERN_ERR "BIOS bug, local APIC #%d not detected!...\n",
                        boot_cpu_id);
                printk(KERN_ERR "... forcing use of dummy APIC emulation. (tell your hw vendor)\n");
                nr_ioapics = 0;
                return -1;
        }

        /*
         * If SMP should be disabled, then really disable it!
         */
        if (!max_cpus) {
                printk(KERN_INFO "SMP mode deactivated, forcing use of dummy APIC emulation.\n");
                nr_ioapics = 0;
                return -1;
        }

        return 0;
}

/*
 * Prepare for SMP bootup.  The MP table or ACPI has been read
 * earlier.  Just do some sanity checking here and enable APIC mode.
 */
void __init smp_prepare_cpus(unsigned int max_cpus)
{
        nmi_watchdog_default();
        current_cpu_data = boot_cpu_data;
        current_thread_info()->cpu = 0;  /* needed? */
        set_cpu_sibling_map(0);

        if (smp_sanity_check(max_cpus) < 0) {
                printk(KERN_INFO "SMP disabled\n");
                disable_smp();
                return;
        }


        /*
         * Switch from PIC to APIC mode.
         */
        setup_local_APIC();

        if (GET_APIC_ID(apic_read(APIC_ID)) != boot_cpu_id) {
                panic("Boot APIC ID in local APIC unexpected (%d vs %d)",
                      GET_APIC_ID(apic_read(APIC_ID)), boot_cpu_id);
                /* Or can we switch back to PIC here? */
        }

        /*
         * Now start the IO-APICs
         */
        if (!skip_ioapic_setup && nr_ioapics)
                setup_IO_APIC();
        else
                nr_ioapics = 0;

        /*
         * Set up local APIC timer on boot CPU.
         */

        setup_boot_APIC_clock();
}

/*
 * Early setup to make printk work.
 */
void __init smp_prepare_boot_cpu(void)
{
        int me = smp_processor_id();
        cpu_set(me, cpu_online_map);
        cpu_set(me, cpu_callout_map);
        per_cpu(cpu_state, me) = CPU_ONLINE;
}

/*
 * Entry point to boot a CPU.
 */
int __cpuinit __cpu_up(unsigned int cpu)
{
        int apicid = cpu_present_to_apicid(cpu);
        unsigned long flags;
        int err;

        WARN_ON(irqs_disabled());

        Dprintk("++++++++++++++++++++=_---CPU UP  %u\n", cpu);

        if (apicid == BAD_APICID || apicid == boot_cpu_id ||
            !physid_isset(apicid, phys_cpu_present_map)) {
                printk("__cpu_up: bad cpu %d\n", cpu);
                return -EINVAL;
        }

        /*
         * Already booted CPU?
         */
        if (cpu_isset(cpu, cpu_callin_map)) {
                Dprintk("do_boot_cpu %d Already started\n", cpu);
                return -ENOSYS;
        }

        /*
         * Save current MTRR state in case it was changed since early boot
         * (e.g. by the ACPI SMI) to initialize new CPUs with MTRRs in sync:
         */
        mtrr_save_state();

        per_cpu(cpu_state, cpu) = CPU_UP_PREPARE;
        /* Boot it! */
        err = do_boot_cpu(cpu, apicid);
        if (err < 0) {
                Dprintk("do_boot_cpu failed %d\n", err);
                return err;
        }

        /* Unleash the CPU! */
        Dprintk("waiting for cpu %d\n", cpu);

        /*
         * Make sure and check TSC sync:
         */
        local_irq_save(flags);
        check_tsc_sync_source(cpu);
        local_irq_restore(flags);

        while (!cpu_isset(cpu, cpu_online_map))
                cpu_relax();
        err = 0;

        return err;
}

/*
 * Finish the SMP boot.
 */
void __init smp_cpus_done(unsigned int max_cpus)
{
        smp_cleanup_boot();
        setup_ioapic_dest();
        check_nmi_watchdog();
}

#ifdef CONFIG_HOTPLUG_CPU

static void remove_siblinginfo(int cpu)
{
        int sibling;
        struct cpuinfo_x86 *c = cpu_data;

        for_each_cpu_mask(sibling, cpu_core_map[cpu]) {
                cpu_clear(cpu, cpu_core_map[sibling]);
                /*
                 * last thread sibling in this cpu core going down
                 */
                if (cpus_weight(cpu_sibling_map[cpu]) == 1)
                        c[sibling].booted_cores--;
        }
                        
        for_each_cpu_mask(sibling, cpu_sibling_map[cpu])
                cpu_clear(cpu, cpu_sibling_map[sibling]);
        cpus_clear(cpu_sibling_map[cpu]);
        cpus_clear(cpu_core_map[cpu]);
        c[cpu].phys_proc_id = 0;
        c[cpu].cpu_core_id = 0;
        cpu_clear(cpu, cpu_sibling_setup_map);
}

void remove_cpu_from_maps(void)
{
        int cpu = smp_processor_id();

        cpu_clear(cpu, cpu_callout_map);
        cpu_clear(cpu, cpu_callin_map);
        clear_bit(cpu, &cpu_initialized); /* was set by cpu_init() */
        clear_node_cpumask(cpu);
}

int __cpu_disable(void)
{
        int cpu = smp_processor_id();

        /*
         * Perhaps use cpufreq to drop frequency, but that could go
         * into generic code.
         *
         * We won't take down the boot processor on i386 due to some
         * interrupts only being able to be serviced by the BSP.
         * Especially so if we're not using an IOAPIC   -zwane
         */
        if (cpu == 0)
                return -EBUSY;

        if (nmi_watchdog == NMI_LOCAL_APIC)
                stop_apic_nmi_watchdog(NULL);
        clear_local_APIC();

        /*
         * HACK:
         * Allow any queued timer interrupts to get serviced
         * This is only a temporary solution until we cleanup
         * fixup_irqs as we do for IA64.
         */
        local_irq_enable();
        mdelay(1);

        local_irq_disable();
        remove_siblinginfo(cpu);

        spin_lock(&vector_lock);
        /* It's now safe to remove this processor from the online map */
        cpu_clear(cpu, cpu_online_map);
        spin_unlock(&vector_lock);
        remove_cpu_from_maps();
        fixup_irqs(cpu_online_map);
        return 0;
}

void __cpu_die(unsigned int cpu)
{
        /* We don't do anything here: idle task is faking death itself. */
        unsigned int i;

        for (i = 0; i < 10; i++) {
                /* They ack this in play_dead by setting CPU_DEAD */
                if (per_cpu(cpu_state, cpu) == CPU_DEAD) {
                        printk ("CPU %d is now offline\n", cpu);
                        if (1 == num_online_cpus())
                                alternatives_smp_switch(0);
                        return;
                }
                msleep(100);
        }
        printk(KERN_ERR "CPU %u didn't die...\n", cpu);
}

static __init int setup_additional_cpus(char *s)
{
        return s && get_option(&s, &additional_cpus) ? 0 : -EINVAL;
}
early_param("additional_cpus", setup_additional_cpus);

#else /* ... !CONFIG_HOTPLUG_CPU */

int __cpu_disable(void)
{
        return -ENOSYS;
}

void __cpu_die(unsigned int cpu)
{
        /* We said "no" in __cpu_disable */
        BUG();
}
#endif /* CONFIG_HOTPLUG_CPU */