// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * 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 <benh@kernel.crashing.org>
 *                IBM Corp.
 *
 *  Derived from previous arch/powerpc/mm/mmu_context.c
 *  and arch/powerpc/include/asm/mmu_context.h
 *
 * 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
 */

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

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

#include <mm/mmu_decl.h>

/*
 * Room for two PTE table pointers, usually the kernel and current user
 * pointer to their respective root page table (pgdir).
 */
void *abatron_pteptrs[2];

/*
 * 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
 */
#define FIRST_CONTEXT 1
#if defined(CONFIG_PPC_8xx)
#define LAST_CONTEXT 16
#elif defined(CONFIG_PPC_47x)
#define LAST_CONTEXT 65535
#else
#define LAST_CONTEXT 255
#endif

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
 */
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;
                }

                /* 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++) {
                                if (stale_map[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;
}

static unsigned int steal_all_contexts(void)
{
        struct mm_struct *mm;
        int cpu = smp_processor_id();
        unsigned int id;

        for (id = FIRST_CONTEXT; id <= LAST_CONTEXT; id++) {
                /* Pick up the victim mm */
                mm = context_mm[id];

                /* Mark this mm as having no context anymore */
                mm->context.id = MMU_NO_CONTEXT;
                if (id != FIRST_CONTEXT) {
                        context_mm[id] = NULL;
                        __clear_bit(id, context_map);
                }
                if (IS_ENABLED(CONFIG_SMP))
                        __clear_bit(id, stale_map[cpu]);
        }

        /* Flush the TLB for all contexts (not to be used on SMP) */
        _tlbil_all();

        nr_free_contexts = LAST_CONTEXT - FIRST_CONTEXT;

        return FIRST_CONTEXT;
}

/* 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];

        /* 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 */
        if (IS_ENABLED(CONFIG_SMP))
                __clear_bit(id, stale_map[cpu]);

        return id;
}

static void set_context(unsigned long id, pgd_t *pgd)
{
        if (IS_ENABLED(CONFIG_PPC_8xx)) {
                mtspr(SPRN_M_TWB, __pa(pgd));

                /* Update context */
                mtspr(SPRN_M_CASID, id - 1);

                /* sync */
                mb();
        } else if (kuap_is_disabled()) {
                mtspr(SPRN_PID, id);
                isync();
        }
}

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

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

        if (IS_ENABLED(CONFIG_SMP)) {
                /* Mark us active and the previous one not anymore */
                next->context.active++;
                if (prev) {
                        WARN_ON(prev->context.active < 1);
                        prev->context.active--;
                }
        }

 again:

        /* If we already have a valid assigned context, skip all that */
        id = next->context.id;
        if (likely(id != MMU_NO_CONTEXT))
                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) {
                if (num_online_cpus() > 1) {
                        id = steal_context_smp(id);
                        if (id == MMU_NO_CONTEXT)
                                goto again;
                        goto stolen;
                }
                if (IS_ENABLED(CONFIG_PPC_8xx))
                        id = steal_all_contexts();
                else
                        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;

 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 (IS_ENABLED(CONFIG_SMP) && test_bit(id, stale_map[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++) {
                        if (stale_map[i])
                                __clear_bit(id, stale_map[i]);
                }
        }

        /* Flick the MMU and release lock */
        if (IS_ENABLED(CONFIG_BDI_SWITCH))
                abatron_pteptrs[1] = next->pgd;
        set_context(id, next->pgd);
#if defined(CONFIG_BOOKE) && defined(CONFIG_PPC_KUAP)
        tsk->thread.pid = id;
#endif
        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)
{
        mm->context.id = MMU_NO_CONTEXT;
        mm->context.active = 0;
        pte_frag_set(&mm->context, NULL);
        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;
                context_mm[id] = NULL;
                nr_free_contexts++;
        }
        raw_spin_unlock_irqrestore(&context_lock, flags);
}

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

        stale_map[cpu] = kzalloc(CTX_MAP_SIZE, GFP_KERNEL);
        return 0;
}

static int mmu_ctx_cpu_dead(unsigned int cpu)
{
#ifdef CONFIG_HOTPLUG_CPU
        if (cpu == boot_cpuid)
                return 0;

        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);
#endif
        return 0;
}

/*
 * 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;

        /*
         * Allocate the maps used by context management
         */
        context_map = memblock_alloc_or_panic(CTX_MAP_SIZE, SMP_CACHE_BYTES);
        context_mm = memblock_alloc_or_panic(sizeof(void *) * (LAST_CONTEXT + 1),
                                    SMP_CACHE_BYTES);
        if (IS_ENABLED(CONFIG_SMP)) {
                stale_map[boot_cpuid] = memblock_alloc_or_panic(CTX_MAP_SIZE, SMP_CACHE_BYTES);
                cpuhp_setup_state_nocalls(CPUHP_POWERPC_MMU_CTX_PREPARE,
                                          "powerpc/mmu/ctx:prepare",
                                          mmu_ctx_cpu_prepare, mmu_ctx_cpu_dead);
        }

        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;
}