ARC: [ASID] get_new_mmu_context() to conditionally allocate new ASID

ASID allocation changes/1

This patch does 2 things:

(1) get_new_mmu_context() NOW moves mm->ASID to a new value ONLY if it
    was from a prev allocation cycle/generation OR if mm had no ASID
    allocated (vs. before would unconditionally moving to a new ASID)

    Callers desiring unconditional update of ASID, e.g.local_flush_tlb_mm()
    (for parent's address space invalidation at fork) need to first force
    the parent to an unallocated ASID.

(2) get_new_mmu_context() always sets the MMU PID reg with unchanged/new
    ASID value.

The gains are:
- consolidation of all asid alloc logic into get_new_mmu_context()
- avoiding code duplication in switch_mm() for PID reg setting
- Enables future change to fold activate_mm() into switch_mm()

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

1 files changed, 18 insertions, 27 deletions

diff --git a/arch/arc/include/asm/mmu_context.h b/arch/arc/include/asm/mmu_context.h
index a63800fd1dba..7a3ecd25ffc9 100644
--- a/arch/arc/include/asm/mmu_context.h
+++ b/arch/arc/include/asm/mmu_context.h
@@ -69,8 +69,8 @@ extern struct mm_struct *asid_mm_map[NUM_ASID + 1];
 extern int asid_cache;
 
 /*
- * Assign a new ASID to task. If the task already has an ASID, it is
- * relinquished.
+ * Get a new ASID if task doesn't have a valid one (unalloc or from prev cycle)
+ * Also set the MMU PID register to existing/updated ASID
  */
 static inline void get_new_mmu_context(struct mm_struct *mm)
 {
@@ -80,6 +80,17 @@ static inline void get_new_mmu_context(struct mm_struct *mm)
         local_irq_save(flags);
 
         /*
+         * Move to new ASID if it was not from current alloc-cycle/generation.
+         *
+         * Note: Callers needing new ASID unconditionally, independent of
+         *       generation, e.g. local_flush_tlb_mm() for forking  parent,
+         *       first need to destroy the context, setting it to invalid
+         *       value.
+         */
+        if (mm->context.asid <= asid_cache)
+                goto set_hw;
+
+        /*
          * Relinquish the currently owned ASID (if any).
          * Doing unconditionally saves a cmp-n-branch; for already unused
          * ASID slot, the value was/remains NULL
@@ -99,9 +110,9 @@ static inline void get_new_mmu_context(struct mm_struct *mm)
          * task with ASID from prev allocation cycle (before ASID roll-over).
          *
          * This might look wrong - if we are re-using some other task's ASID,
-         * won't we use it's stale TLB entries too. Actually switch_mm( ) takes
+         * won't we use it's stale TLB entries too. Actually the algorithm takes
          * care of such a case: it ensures that task with ASID from prev alloc
-         * cycle, when scheduled will refresh it's ASID: see switch_mm( ) below
+         * cycle, when scheduled will refresh it's ASID
          * The stealing scenario described here will only happen if that task
          * didn't get a chance to refresh it's ASID - implying stale entries
          * won't exist.
@@ -114,7 +125,8 @@ static inline void get_new_mmu_context(struct mm_struct *mm)
         asid_mm_map[asid_cache] = mm;
         mm->context.asid = asid_cache;
 
-        write_aux_reg(ARC_REG_PID, asid_cache | MMU_ENABLE);
+set_hw:
+        write_aux_reg(ARC_REG_PID, mm->context.asid | MMU_ENABLE);
 
         local_irq_restore(flags);
 }
@@ -141,28 +153,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
         write_aux_reg(ARC_REG_SCRATCH_DATA0, next->pgd);
 #endif
 
-        /*
-         * Get a new ASID if task doesn't have a valid one. Possible when
-         *  -task never had an ASID (fresh after fork)
-         *  -it's ASID was stolen - past an ASID roll-over.
-         *  -There's a third obscure scenario (if this task is running for the
-         *   first time afer an ASID rollover), where despite having a valid
-         *   ASID, we force a get for new ASID - see comments at top.
-         *
-         * Both the non-alloc scenario and first-use-after-rollover can be
-         * detected using the single condition below:  NO_ASID = 256
-         * while asid_cache is always a valid ASID value (0-255).
-         */
-        if (next->context.asid > asid_cache) {
-                get_new_mmu_context(next);
-        } else {
-                /*
-                 * XXX: This will never happen given the chks above
-                 * BUG_ON(next->context.asid > MAX_ASID);
-                 */
-                write_aux_reg(ARC_REG_PID, next->context.asid | MMU_ENABLE);
-        }
-
+        get_new_mmu_context(next);
 }
 
 static inline void destroy_context(struct mm_struct *mm)