arch/arc/include/asm/mmu_context.h

   1 /*
   2  * Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License version 2 as
   6  * published by the Free Software Foundation.
   7  *
   8  * vineetg: May 2011
   9  *  -Refactored get_new_mmu_context( ) to only handle live-mm.
  10  *   retiring-mm handled in other hooks
  11  *
  12  * Vineetg: March 25th, 2008: Bug #92690
  13  *  -Major rewrite of Core ASID allocation routine get_new_mmu_context
  14  *
  15  * Amit Bhor, Sameer Dhavale: Codito Technologies 2004
  16  */
  17
  18 #ifndef _ASM_ARC_MMU_CONTEXT_H
  19 #define _ASM_ARC_MMU_CONTEXT_H
  20
  21 #include <asm/arcregs.h>
  22 #include <asm/tlb.h>
  23
  24 #include <asm-generic/mm_hooks.h>
  25
  26 /*              ARC700 ASID Management
  27  *
  28  * ARC MMU provides 8-bit ASID (0..255) to TAG TLB entries, allowing entries
  29  * with same vaddr (different tasks) to co-exit. This provides for
  30  * "Fast Context Switch" i.e. no TLB flush on ctxt-switch
  31  *
  32  * Linux assigns each task a unique ASID. A simple round-robin allocation
  33  * of H/w ASID is done using software tracker @asid_cache.
  34  * When it reaches max 255, the allocation cycle starts afresh by flushing
  35  * the entire TLB and wrapping ASID back to zero.
  36  *
  37  * For book-keeping, Linux uses a couple of data-structures:
  38  *  -mm_struct has an @asid field to keep a note of task's ASID (needed at the
  39  *   time of say switch_mm( )
  40  *  -An array of mm structs @asid_mm_map[] for asid->mm the reverse mapping,
  41  *  given an ASID, finding the mm struct associated.
  42  *
  43  * The round-robin allocation algorithm allows for ASID stealing.
  44  * If asid tracker is at "x-1", a new req will allocate "x", even if "x" was
  45  * already assigned to another (switched-out) task. Obviously the prev owner
  46  * is marked with an invalid ASID to make it request for a new ASID when it
  47  * gets scheduled next time. However its TLB entries (with ASID "x") could
  48  * exist, which must be cleared before the same ASID is used by the new owner.
  49  * Flushing them would be plausible but costly solution. Instead we force a
  50  * allocation policy quirk, which ensures that a stolen ASID won't have any
  51  * TLB entries associates, alleviating the need to flush.
  52  * The quirk essentially is not allowing ASID allocated in prev cycle
  53  * to be used past a roll-over in the next cycle.
  54  * When this happens (i.e. task ASID > asid tracker), task needs to refresh
  55  * its ASID, aligning it to current value of tracker. If the task doesn't get
  56  * scheduled past a roll-over, hence its ASID is not yet realigned with
  57  * tracker, such ASID is anyways safely reusable because it is
  58  * gauranteed that TLB entries with that ASID wont exist.
  59  */
  60
  61 #define FIRST_ASID  0
  62 #define MAX_ASID    255                 /* 8 bit PID field in PID Aux reg */
  63 #define NO_ASID     (MAX_ASID + 1)      /* ASID Not alloc to mmu ctxt */
  64 #define NUM_ASID    ((MAX_ASID - FIRST_ASID) + 1)
  65
  66 /* ASID to mm struct mapping */
  67 extern struct mm_struct *asid_mm_map[NUM_ASID + 1];
  68
  69 extern int asid_cache;
  70
  71 /*
  72  * Assign a new ASID to task. If the task already has an ASID, it is
  73  * relinquished.
  74  */
  75 static inline void get_new_mmu_context(struct mm_struct *mm)
  76 {
  77         struct mm_struct *prev_owner;
  78         unsigned long flags;
  79
  80         local_irq_save(flags);
  81
  82         /*
  83          * Relinquish the currently owned ASID (if any).
  84          * Doing unconditionally saves a cmp-n-branch; for already unused
  85          * ASID slot, the value was/remains NULL
  86          */
  87         asid_mm_map[mm->context.asid] = (struct mm_struct *)NULL;
  88
  89         /* move to new ASID */
  90         if (++asid_cache > MAX_ASID) {  /* ASID roll-over */
  91                 asid_cache = FIRST_ASID;
  92                 flush_tlb_all();
  93         }
  94
  95         /*
  96          * Is next ASID already owned by some-one else (we are stealing it).
  97          * If so, let the orig owner be aware of this, so when it runs, it
  98          * asks for a brand new ASID. This would only happen for a long-lived
  99          * task with ASID from prev allocation cycle (before ASID roll-over).
 100          *
 101          * This might look wrong - if we are re-using some other task's ASID,
 102          * won't we use it's stale TLB entries too. Actually switch_mm( ) takes
 103          * care of such a case: it ensures that task with ASID from prev alloc
 104          * cycle, when scheduled will refresh it's ASID: see switch_mm( ) below
 105          * The stealing scenario described here will only happen if that task
 106          * didn't get a chance to refresh it's ASID - implying stale entries
 107          * won't exist.
 108          */
 109         prev_owner = asid_mm_map[asid_cache];
 110         if (prev_owner)
 111                 prev_owner->context.asid = NO_ASID;
 112
 113         /* Assign new ASID to tsk */
 114         asid_mm_map[asid_cache] = mm;
 115         mm->context.asid = asid_cache;
 116
 117 #ifdef CONFIG_ARC_TLB_DBG
 118         pr_info("ARC_TLB_DBG: NewMM=0x%x OldMM=0x%x task_struct=0x%x Task: %s,"
 119                " pid:%u, assigned asid:%lu\n",
 120                (unsigned int)mm, (unsigned int)prev_owner,
 121                (unsigned int)(mm->context.tsk), (mm->context.tsk)->comm,
 122                (mm->context.tsk)->pid, mm->context.asid);
 123 #endif
 124
 125         write_aux_reg(ARC_REG_PID, asid_cache | MMU_ENABLE);
 126
 127         local_irq_restore(flags);
 128 }
 129
 130 /*
 131  * Initialize the context related info for a new mm_struct
 132  * instance.
 133  */
 134 static inline int
 135 init_new_context(struct task_struct *tsk, struct mm_struct *mm)
 136 {
 137         mm->context.asid = NO_ASID;
 138 #ifdef CONFIG_ARC_TLB_DBG
 139         mm->context.tsk = tsk;
 140 #endif
 141         return 0;
 142 }
 143
 144 /* Prepare the MMU for task: setup PID reg with allocated ASID
 145     If task doesn't have an ASID (never alloc or stolen, get a new ASID)
 146 */
 147 static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 148                              struct task_struct *tsk)
 149 {
 150 #ifndef CONFIG_SMP
 151         /* PGD cached in MMU reg to avoid 3 mem lookups: task->mm->pgd */
 152         write_aux_reg(ARC_REG_SCRATCH_DATA0, next->pgd);
 153 #endif
 154
 155         /*
 156          * Get a new ASID if task doesn't have a valid one. Possible when
 157          *  -task never had an ASID (fresh after fork)
 158          *  -it's ASID was stolen - past an ASID roll-over.
 159          *  -There's a third obscure scenario (if this task is running for the
 160          *   first time afer an ASID rollover), where despite having a valid
 161          *   ASID, we force a get for new ASID - see comments at top.
 162          *
 163          * Both the non-alloc scenario and first-use-after-rollover can be
 164          * detected using the single condition below:  NO_ASID = 256
 165          * while asid_cache is always a valid ASID value (0-255).
 166          */
 167         if (next->context.asid > asid_cache) {
 168                 get_new_mmu_context(next);
 169         } else {
 170                 /*
 171                  * XXX: This will never happen given the chks above
 172                  * BUG_ON(next->context.asid > MAX_ASID);
 173                  */
 174                 write_aux_reg(ARC_REG_PID, next->context.asid | MMU_ENABLE);
 175         }
 176
 177 }
 178
 179 static inline void destroy_context(struct mm_struct *mm)
 180 {
 181         unsigned long flags;
 182
 183         local_irq_save(flags);
 184
 185         asid_mm_map[mm->context.asid] = NULL;
 186         mm->context.asid = NO_ASID;
 187
 188         local_irq_restore(flags);
 189 }
 190
 191 /* it seemed that deactivate_mm( ) is a reasonable place to do book-keeping
 192  * for retiring-mm. However destroy_context( ) still needs to do that because
 193  * between mm_release( ) = >deactive_mm( ) and
 194  * mmput => .. => __mmdrop( ) => destroy_context( )
 195  * there is a good chance that task gets sched-out/in, making it's ASID valid
 196  * again (this teased me for a whole day).
 197  */
 198 #define deactivate_mm(tsk, mm)   do { } while (0)
 199
 200 static inline void activate_mm(struct mm_struct *prev, struct mm_struct *next)
 201 {
 202 #ifndef CONFIG_SMP
 203         write_aux_reg(ARC_REG_SCRATCH_DATA0, next->pgd);
 204 #endif
 205
 206         /* Unconditionally get a new ASID */
 207         get_new_mmu_context(next);
 208
 209 }
 210
 211 #define enter_lazy_tlb(mm, tsk)
 212
 213 #endif /* __ASM_ARC_MMU_CONTEXT_H */