mmu_context_nohash.c

Go to the documentation of this file.

/*
 * This file contains the routines for handling the MMU on those
 * PowerPC implementations where the MMU is not using the hash
 * table, such as 8xx, 4xx, BookE's etc...
 *
 * Copyright 2008 Ben Herrenschmidt <[email protected]>
 *                IBM Corp.
 *
 *  Derived from previous arch/powerpc/mm/mmu_context.c
 *  and arch/powerpc/include/asm/mmu_context.h
 *
 *  This program is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU General Public License
 *  as published by the Free Software Foundation; either version
 *  2 of the License, or (at your option) any later version.
 *
 * TODO:
 *
 *   - The global context lock will not scale very well
 *   - The maps should be dynamically allocated to allow for processors
 *     that support more PID bits at runtime
 *   - Implement flush_tlb_mm() by making the context stale and picking
 *     a new one
 *   - More aggressively clear stale map bits and maybe find some way to
 *     also clear mm->cpu_vm_mask bits when processes are migrated
 */

//#define DEBUG_MAP_CONSISTENCY
//#define DEBUG_CLAMP_LAST_CONTEXT   31
//#define DEBUG_HARDER

/* We don't use DEBUG because it tends to be compiled in always nowadays
 * and this would generate way too much output
 */
#ifdef DEBUG_HARDER
#define pr_hard(args...)    printk(KERN_DEBUG args)
#define pr_hardcont(args...)    printk(KERN_CONT args)
#else
#define pr_hard(args...)    do { } while(0)
#define pr_hardcont(args...)    do { } while(0)
#endif

#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/init.h>
#include <linux/spinlock.h>
#include <linux/bootmem.h>
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/slab.h>

#include <asm/mmu_context.h>
#include <asm/tlbflush.h>

static unsigned int first_context, last_context;
static unsigned int next_context, nr_free_contexts;
static unsigned long *context_map;
static unsigned long *stale_map[NR_CPUS];
static struct mm_struct **context_mm;
static DEFINE_RAW_SPINLOCK(context_lock);

#define CTX_MAP_SIZE    \
    (sizeof(unsigned long) * (last_context / BITS_PER_LONG + 1))


/* Steal a context from a task that has one at the moment.
 *
 * This is used when we are running out of available PID numbers
 * on the processors.
 *
 * This isn't an LRU system, it just frees up each context in
 * turn (sort-of pseudo-random replacement :).  This would be the
 * place to implement an LRU scheme if anyone was motivated to do it.
 *  -- paulus
 *
 * For context stealing, we use a slightly different approach for
 * SMP and UP. Basically, the UP one is simpler and doesn't use
 * the stale map as we can just flush the local CPU
 *  -- benh
 */
#ifdef CONFIG_SMP
static unsigned int steal_context_smp(unsigned int id)
{
    struct mm_struct *mm;
    unsigned int cpu, max, i;

    max = last_context - first_context;

    /* Attempt to free next_context first and then loop until we manage */
    while (max--) {
        /* Pick up the victim mm */
        mm = context_mm[id];

        /* We have a candidate victim, check if it's active, on SMP
         * we cannot steal active contexts
         */
        if (mm->context.active) {
            id++;
            if (id > last_context)
                id = first_context;
            continue;
        }
        pr_hardcont(" | steal %d from 0x%p", id, mm);

        /* Mark this mm has having no context anymore */
        mm->context.id = MMU_NO_CONTEXT;

        /* Mark it stale on all CPUs that used this mm. For threaded
         * implementations, we set it on all threads on each core
         * represented in the mask. A future implementation will use
         * a core map instead but this will do for now.
         */
        for_each_cpu(cpu, mm_cpumask(mm)) {
            for (i = cpu_first_thread_sibling(cpu);
                 i <= cpu_last_thread_sibling(cpu); i++)
                __set_bit(id, stale_map[i]);
            cpu = i - 1;
        }
        return id;
    }

    /* This will happen if you have more CPUs than available contexts,
     * all we can do here is wait a bit and try again
     */
    raw_spin_unlock(&context_lock);
    cpu_relax();
    raw_spin_lock(&context_lock);

    /* This will cause the caller to try again */
    return MMU_NO_CONTEXT;
}
#endif  /* CONFIG_SMP */

/* Note that this will also be called on SMP if all other CPUs are
 * offlined, which means that it may be called for cpu != 0. For
 * this to work, we somewhat assume that CPUs that are onlined
 * come up with a fully clean TLB (or are cleaned when offlined)
 */
static unsigned int steal_context_up(unsigned int id)
{
    struct mm_struct *mm;
    int cpu = smp_processor_id();

    /* Pick up the victim mm */
    mm = context_mm[id];

    pr_hardcont(" | steal %d from 0x%p", id, mm);

    /* Flush the TLB for that context */
    local_flush_tlb_mm(mm);

    /* Mark this mm has having no context anymore */
    mm->context.id = MMU_NO_CONTEXT;

    /* XXX This clear should ultimately be part of local_flush_tlb_mm */
    __clear_bit(id, stale_map[cpu]);

    return id;
}

#ifdef DEBUG_MAP_CONSISTENCY
static void context_check_map(void)
{
    unsigned int id, nrf, nact;

    nrf = nact = 0;
    for (id = first_context; id <= last_context; id++) {
        int used = test_bit(id, context_map);
        if (!used)
            nrf++;
        if (used != (context_mm[id] != NULL))
            pr_err("MMU: Context %d is %s and MM is %p !\n",
                   id, used ? "used" : "free", context_mm[id]);
        if (context_mm[id] != NULL)
            nact += context_mm[id]->context.active;
    }
    if (nrf != nr_free_contexts) {
        pr_err("MMU: Free context count out of sync ! (%d vs %d)\n",
               nr_free_contexts, nrf);
        nr_free_contexts = nrf;
    }
    if (nact > num_online_cpus())
        pr_err("MMU: More active contexts than CPUs ! (%d vs %d)\n",
               nact, num_online_cpus());
    if (first_context > 0 && !test_bit(0, context_map))
        pr_err("MMU: Context 0 has been freed !!!\n");
}
#else
static void context_check_map(void) { }
#endif

void switch_mmu_context(struct mm_struct *prev, struct mm_struct *next)
{
    unsigned int i, id, cpu = smp_processor_id();
    unsigned long *map;

    /* No lockless fast path .. yet */
    raw_spin_lock(&context_lock);

    pr_hard("[%d] activating context for mm @%p, active=%d, id=%d",
        cpu, next, next->context.active, next->context.id);

#ifdef CONFIG_SMP
    /* Mark us active and the previous one not anymore */
    next->context.active++;
    if (prev) {
        pr_hardcont(" (old=0x%p a=%d)", prev, prev->context.active);
        WARN_ON(prev->context.active < 1);
        prev->context.active--;
    }

 again:
#endif /* CONFIG_SMP */

    /* If we already have a valid assigned context, skip all that */
    id = next->context.id;
    if (likely(id != MMU_NO_CONTEXT)) {
#ifdef DEBUG_MAP_CONSISTENCY
        if (context_mm[id] != next)
            pr_err("MMU: mm 0x%p has id %d but context_mm[%d] says 0x%p\n",
                   next, id, id, context_mm[id]);
#endif
        goto ctxt_ok;
    }

    /* We really don't have a context, let's try to acquire one */
    id = next_context;
    if (id > last_context)
        id = first_context;
    map = context_map;

    /* No more free contexts, let's try to steal one */
    if (nr_free_contexts == 0) {
#ifdef CONFIG_SMP
        if (num_online_cpus() > 1) {
            id = steal_context_smp(id);
            if (id == MMU_NO_CONTEXT)
                goto again;
            goto stolen;
        }
#endif /* CONFIG_SMP */
        id = steal_context_up(id);
        goto stolen;
    }
    nr_free_contexts--;

    /* We know there's at least one free context, try to find it */
    while (__test_and_set_bit(id, map)) {
        id = find_next_zero_bit(map, last_context+1, id);
        if (id > last_context)
            id = first_context;
    }
 stolen:
    next_context = id + 1;
    context_mm[id] = next;
    next->context.id = id;
    pr_hardcont(" | new id=%d,nrf=%d", id, nr_free_contexts);

    context_check_map();
 ctxt_ok:

    /* If that context got marked stale on this CPU, then flush the
     * local TLB for it and unmark it before we use it
     */
    if (test_bit(id, stale_map[cpu])) {
        pr_hardcont(" | stale flush %d [%d..%d]",
                id, cpu_first_thread_sibling(cpu),
                cpu_last_thread_sibling(cpu));

        local_flush_tlb_mm(next);

        /* XXX This clear should ultimately be part of local_flush_tlb_mm */
        for (i = cpu_first_thread_sibling(cpu);
             i <= cpu_last_thread_sibling(cpu); i++) {
            __clear_bit(id, stale_map[i]);
        }
    }

    /* Flick the MMU and release lock */
    pr_hardcont(" -> %d\n", id);
    set_context(id, next->pgd);
    raw_spin_unlock(&context_lock);
}

/*
 * Set up the context for a new address space.
 */
int init_new_context(struct task_struct *t, struct mm_struct *mm)
{
    pr_hard("initing context for mm @%p\n", mm);

    mm->context.id = MMU_NO_CONTEXT;
    mm->context.active = 0;

#ifdef CONFIG_PPC_MM_SLICES
    if (slice_mm_new_context(mm))
        slice_set_user_psize(mm, mmu_virtual_psize);
#endif

    return 0;
}

/*
 * We're finished using the context for an address space.
 */
void destroy_context(struct mm_struct *mm)
{
    unsigned long flags;
    unsigned int id;

    if (mm->context.id == MMU_NO_CONTEXT)
        return;

    WARN_ON(mm->context.active != 0);

    raw_spin_lock_irqsave(&context_lock, flags);
    id = mm->context.id;
    if (id != MMU_NO_CONTEXT) {
        __clear_bit(id, context_map);
        mm->context.id = MMU_NO_CONTEXT;
#ifdef DEBUG_MAP_CONSISTENCY
        mm->context.active = 0;
#endif
        context_mm[id] = NULL;
        nr_free_contexts++;
    }
    raw_spin_unlock_irqrestore(&context_lock, flags);
}

#ifdef CONFIG_SMP

static int __cpuinit mmu_context_cpu_notify(struct notifier_block *self,
                        unsigned long action, void *hcpu)
{
    unsigned int cpu = (unsigned int)(long)hcpu;

    /* We don't touch CPU 0 map, it's allocated at aboot and kept
     * around forever
     */
    if (cpu == boot_cpuid)
        return NOTIFY_OK;

    switch (action) {
    case CPU_UP_PREPARE:
    case CPU_UP_PREPARE_FROZEN:
        pr_devel("MMU: Allocating stale context map for CPU %d\n", cpu);
        stale_map[cpu] = kzalloc(CTX_MAP_SIZE, GFP_KERNEL);
        break;
#ifdef CONFIG_HOTPLUG_CPU
    case CPU_UP_CANCELED:
    case CPU_UP_CANCELED_FROZEN:
    case CPU_DEAD:
    case CPU_DEAD_FROZEN:
        pr_devel("MMU: Freeing stale context map for CPU %d\n", cpu);
        kfree(stale_map[cpu]);
        stale_map[cpu] = NULL;

        /* We also clear the cpu_vm_mask bits of CPUs going away */
        clear_tasks_mm_cpumask(cpu);
    break;
#endif /* CONFIG_HOTPLUG_CPU */
    }
    return NOTIFY_OK;
}

static struct notifier_block __cpuinitdata mmu_context_cpu_nb = {
    .notifier_call  = mmu_context_cpu_notify,
};

#endif /* CONFIG_SMP */

/*
 * Initialize the context management stuff.
 */
void __init mmu_context_init(void)
{
    /* Mark init_mm as being active on all possible CPUs since
     * we'll get called with prev == init_mm the first time
     * we schedule on a given CPU
     */
    init_mm.context.active = NR_CPUS;

    /*
     *   The MPC8xx has only 16 contexts.  We rotate through them on each
     * task switch.  A better way would be to keep track of tasks that
     * own contexts, and implement an LRU usage.  That way very active
     * tasks don't always have to pay the TLB reload overhead.  The
     * kernel pages are mapped shared, so the kernel can run on behalf
     * of any task that makes a kernel entry.  Shared does not mean they
     * are not protected, just that the ASID comparison is not performed.
     *      -- Dan
     *
     * The IBM4xx has 256 contexts, so we can just rotate through these
     * as a way of "switching" contexts.  If the TID of the TLB is zero,
     * the PID/TID comparison is disabled, so we can use a TID of zero
     * to represent all kernel pages as shared among all contexts.
     *  -- Dan
     *
     * The IBM 47x core supports 16-bit PIDs, thus 65535 contexts. We
     * should normally never have to steal though the facility is
     * present if needed.
     *      -- BenH
     */
    if (mmu_has_feature(MMU_FTR_TYPE_8xx)) {
        first_context = 0;
        last_context = 15;
    } else if (mmu_has_feature(MMU_FTR_TYPE_47x)) {
        first_context = 1;
        last_context = 65535;
    } else
#ifdef CONFIG_PPC_BOOK3E_MMU
    if (mmu_has_feature(MMU_FTR_TYPE_3E)) {
        u32 mmucfg = mfspr(SPRN_MMUCFG);
        u32 pid_bits = (mmucfg & MMUCFG_PIDSIZE_MASK)
                >> MMUCFG_PIDSIZE_SHIFT;
        first_context = 1;
        last_context = (1UL << (pid_bits + 1)) - 1;
    } else
#endif
    {
        first_context = 1;
        last_context = 255;
    }

#ifdef DEBUG_CLAMP_LAST_CONTEXT
    last_context = DEBUG_CLAMP_LAST_CONTEXT;
#endif
    /*
     * Allocate the maps used by context management
     */
    context_map = alloc_bootmem(CTX_MAP_SIZE);
    context_mm = alloc_bootmem(sizeof(void *) * (last_context + 1));
#ifndef CONFIG_SMP
    stale_map[0] = alloc_bootmem(CTX_MAP_SIZE);
#else
    stale_map[boot_cpuid] = alloc_bootmem(CTX_MAP_SIZE);

    register_cpu_notifier(&mmu_context_cpu_nb);
#endif

    printk(KERN_INFO
           "MMU: Allocated %zu bytes of context maps for %d contexts\n",
           2 * CTX_MAP_SIZE + (sizeof(void *) * (last_context + 1)),
           last_context - first_context + 1);

    /*
     * Some processors have too few contexts to reserve one for
     * init_mm, and require using context 0 for a normal task.
     * Other processors reserve the use of context zero for the kernel.
     * This code assumes first_context < 32.
     */
    context_map[0] = (1 << first_context) - 1;
    next_context = first_context;
    nr_free_contexts = last_context - first_context + 1;
}