arch/powerpc/mm/mmu_context_hash32.c

   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
  28 #include <asm/mmu_context.h>
  29 #include <asm/tlbflush.h>
  30
  31 /*
  32  * On 32-bit PowerPC 6xx/7xx/7xxx CPUs, we use a set of 16 VSIDs
  33  * (virtual segment identifiers) for each context.  Although the
  34  * hardware supports 24-bit VSIDs, and thus >1 million contexts,
  35  * we only use 32,768 of them.  That is ample, since there can be
  36  * at most around 30,000 tasks in the system anyway, and it means
  37  * that we can use a bitmap to indicate which contexts are in use.
  38  * Using a bitmap means that we entirely avoid all of the problems
  39  * that we used to have when the context number overflowed,
  40  * particularly on SMP systems.
  41  *  -- paulus.
  42  */
  43 #define NO_CONTEXT              ((unsigned long) -1)
  44 #define LAST_CONTEXT            32767
  45 #define FIRST_CONTEXT           1
  46
  47 /*
  48  * This function defines the mapping from contexts to VSIDs (virtual
  49  * segment IDs).  We use a skew on both the context and the high 4 bits
  50  * of the 32-bit virtual address (the "effective segment ID") in order
  51  * to spread out the entries in the MMU hash table.  Note, if this
  52  * function is changed then arch/ppc/mm/hashtable.S will have to be
  53  * changed to correspond.
  54  *
  55  *
  56  * CTX_TO_VSID(ctx, va) (((ctx) * (897 * 16) + ((va) >> 28) * 0x111) \
  57  *                               & 0xffffff)
  58  */
  59
  60 static unsigned long next_mmu_context;
  61 static unsigned long context_map[LAST_CONTEXT / BITS_PER_LONG + 1];
  62
  63
  64 /*
  65  * Set up the context for a new address space.
  66  */
  67 int init_new_context(struct task_struct *t, struct mm_struct *mm)
  68 {
  69         unsigned long ctx = next_mmu_context;
  70
  71         while (test_and_set_bit(ctx, context_map)) {
  72                 ctx = find_next_zero_bit(context_map, LAST_CONTEXT+1, ctx);
  73                 if (ctx > LAST_CONTEXT)
  74                         ctx = 0;
  75         }
  76         next_mmu_context = (ctx + 1) & LAST_CONTEXT;
  77         mm->context.id = ctx;
  78
  79         return 0;
  80 }
  81
  82 /*
  83  * We're finished using the context for an address space.
  84  */
  85 void destroy_context(struct mm_struct *mm)
  86 {
  87         preempt_disable();
  88         if (mm->context.id != NO_CONTEXT) {
  89                 clear_bit(mm->context.id, context_map);
  90                 mm->context.id = NO_CONTEXT;
  91         }
  92         preempt_enable();
  93 }
  94
  95 /*
  96  * Initialize the context management stuff.
  97  */
  98 void __init mmu_context_init(void)
  99 {
 100         /* Reserve context 0 for kernel use */
 101         context_map[0] = (1 << FIRST_CONTEXT) - 1;
 102         next_mmu_context = FIRST_CONTEXT;
 103 }