linux/arch/powerpc/mm/mmu_context_hash32.c
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   1/*
   2 * This file contains the routines for handling the MMU on those
   3 * PowerPC implementations where the MMU substantially follows the
   4 * architecture specification.  This includes the 6xx, 7xx, 7xxx,
   5 * 8260, and POWER3 implementations but excludes the 8xx and 4xx.
   6 *  -- paulus
   7 *
   8 *  Derived from arch/ppc/mm/init.c:
   9 *    Copyright (C) 1995-1996 Gary Thomas (gdt@linuxppc.org)
  10 *
  11 *  Modifications by Paul Mackerras (PowerMac) (paulus@cs.anu.edu.au)
  12 *  and Cort Dougan (PReP) (cort@cs.nmt.edu)
  13 *    Copyright (C) 1996 Paul Mackerras
  14 *
  15 *  Derived from "arch/i386/mm/init.c"
  16 *    Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
  17 *
  18 *  This program is free software; you can redistribute it and/or
  19 *  modify it under the terms of the GNU General Public License
  20 *  as published by the Free Software Foundation; either version
  21 *  2 of the License, or (at your option) any later version.
  22 *
  23 */
  24
  25#include <linux/mm.h>
  26#include <linux/init.h>
  27#include <linux/export.h>
  28
  29#include <asm/mmu_context.h>
  30#include <asm/tlbflush.h>
  31
  32/*
  33 * On 32-bit PowerPC 6xx/7xx/7xxx CPUs, we use a set of 16 VSIDs
  34 * (virtual segment identifiers) for each context.  Although the
  35 * hardware supports 24-bit VSIDs, and thus >1 million contexts,
  36 * we only use 32,768 of them.  That is ample, since there can be
  37 * at most around 30,000 tasks in the system anyway, and it means
  38 * that we can use a bitmap to indicate which contexts are in use.
  39 * Using a bitmap means that we entirely avoid all of the problems
  40 * that we used to have when the context number overflowed,
  41 * particularly on SMP systems.
  42 *  -- paulus.
  43 */
  44#define NO_CONTEXT              ((unsigned long) -1)
  45#define LAST_CONTEXT            32767
  46#define FIRST_CONTEXT           1
  47
  48/*
  49 * This function defines the mapping from contexts to VSIDs (virtual
  50 * segment IDs).  We use a skew on both the context and the high 4 bits
  51 * of the 32-bit virtual address (the "effective segment ID") in order
  52 * to spread out the entries in the MMU hash table.  Note, if this
  53 * function is changed then arch/ppc/mm/hashtable.S will have to be
  54 * changed to correspond.
  55 *
  56 *
  57 * CTX_TO_VSID(ctx, va) (((ctx) * (897 * 16) + ((va) >> 28) * 0x111) \
  58 *                               & 0xffffff)
  59 */
  60
  61static unsigned long next_mmu_context;
  62static unsigned long context_map[LAST_CONTEXT / BITS_PER_LONG + 1];
  63
  64unsigned long __init_new_context(void)
  65{
  66        unsigned long ctx = next_mmu_context;
  67
  68        while (test_and_set_bit(ctx, context_map)) {
  69                ctx = find_next_zero_bit(context_map, LAST_CONTEXT+1, ctx);
  70                if (ctx > LAST_CONTEXT)
  71                        ctx = 0;
  72        }
  73        next_mmu_context = (ctx + 1) & LAST_CONTEXT;
  74
  75        return ctx;
  76}
  77EXPORT_SYMBOL_GPL(__init_new_context);
  78
  79/*
  80 * Set up the context for a new address space.
  81 */
  82int init_new_context(struct task_struct *t, struct mm_struct *mm)
  83{
  84        mm->context.id = __init_new_context();
  85
  86        return 0;
  87}
  88
  89/*
  90 * Free a context ID. Make sure to call this with preempt disabled!
  91 */
  92void __destroy_context(unsigned long ctx)
  93{
  94        clear_bit(ctx, context_map);
  95}
  96EXPORT_SYMBOL_GPL(__destroy_context);
  97
  98/*
  99 * We're finished using the context for an address space.
 100 */
 101void destroy_context(struct mm_struct *mm)
 102{
 103        preempt_disable();
 104        if (mm->context.id != NO_CONTEXT) {
 105                __destroy_context(mm->context.id);
 106                mm->context.id = NO_CONTEXT;
 107        }
 108        preempt_enable();
 109}
 110
 111/*
 112 * Initialize the context management stuff.
 113 */
 114void __init mmu_context_init(void)
 115{
 116        /* Reserve context 0 for kernel use */
 117        context_map[0] = (1 << FIRST_CONTEXT) - 1;
 118        next_mmu_context = FIRST_CONTEXT;
 119}
 120