diff options
Diffstat (limited to 'arch/arc/include/asm/mmu_context.h')
| -rw-r--r-- | arch/arc/include/asm/mmu_context.h | 97 |
1 files changed, 28 insertions, 69 deletions
diff --git a/arch/arc/include/asm/mmu_context.h b/arch/arc/include/asm/mmu_context.h index 9b09d18..43a1b51 100644 --- a/arch/arc/include/asm/mmu_context.h +++ b/arch/arc/include/asm/mmu_context.h @@ -34,39 +34,22 @@ * When it reaches max 255, the allocation cycle starts afresh by flushing * the entire TLB and wrapping ASID back to zero. * - * For book-keeping, Linux uses a couple of data-structures: - * -mm_struct has an @asid field to keep a note of task's ASID (needed at the - * time of say switch_mm( ) - * -An array of mm structs @asid_mm_map[] for asid->mm the reverse mapping, - * given an ASID, finding the mm struct associated. - * - * The round-robin allocation algorithm allows for ASID stealing. - * If asid tracker is at "x-1", a new req will allocate "x", even if "x" was - * already assigned to another (switched-out) task. Obviously the prev owner - * is marked with an invalid ASID to make it request for a new ASID when it - * gets scheduled next time. However its TLB entries (with ASID "x") could - * exist, which must be cleared before the same ASID is used by the new owner. - * Flushing them would be plausible but costly solution. Instead we force a - * allocation policy quirk, which ensures that a stolen ASID won't have any - * TLB entries associates, alleviating the need to flush. - * The quirk essentially is not allowing ASID allocated in prev cycle - * to be used past a roll-over in the next cycle. - * When this happens (i.e. task ASID > asid tracker), task needs to refresh - * its ASID, aligning it to current value of tracker. If the task doesn't get - * scheduled past a roll-over, hence its ASID is not yet realigned with - * tracker, such ASID is anyways safely reusable because it is - * gauranteed that TLB entries with that ASID wont exist. + * A new allocation cycle, post rollover, could potentially reassign an ASID + * to a different task. Thus the rule is to refresh the ASID in a new cycle. + * The 32 bit @asid_cache (and mm->asid) have 8 bits MMU PID and rest 24 bits + * serve as cycle/generation indicator and natural 32 bit unsigned math + * automagically increments the generation when lower 8 bits rollover. */ -#define FIRST_ASID 0 -#define MAX_ASID 255 /* 8 bit PID field in PID Aux reg */ -#define NO_ASID (MAX_ASID + 1) /* ASID Not alloc to mmu ctxt */ -#define NUM_ASID ((MAX_ASID - FIRST_ASID) + 1) +#define MM_CTXT_ASID_MASK 0x000000ff /* MMU PID reg :8 bit PID */ +#define MM_CTXT_CYCLE_MASK (~MM_CTXT_ASID_MASK) + +#define MM_CTXT_FIRST_CYCLE (MM_CTXT_ASID_MASK + 1) +#define MM_CTXT_NO_ASID 0UL -/* ASID to mm struct mapping */ -extern struct mm_struct *asid_mm_map[NUM_ASID + 1]; +#define hw_pid(mm) (mm->context.asid & MM_CTXT_ASID_MASK) -extern int asid_cache; +extern unsigned int asid_cache; /* * Get a new ASID if task doesn't have a valid one (unalloc or from prev cycle) @@ -74,59 +57,42 @@ extern int asid_cache; */ static inline void get_new_mmu_context(struct mm_struct *mm) { - struct mm_struct *prev_owner; unsigned long flags; local_irq_save(flags); /* * Move to new ASID if it was not from current alloc-cycle/generation. + * This is done by ensuring that the generation bits in both mm->ASID + * and cpu's ASID counter are exactly same. * * 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) + if (!((mm->context.asid ^ asid_cache) & MM_CTXT_CYCLE_MASK)) 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 - */ - asid_mm_map[mm->context.asid] = (struct mm_struct *)NULL; + /* move to new ASID and handle rollover */ + if (unlikely(!(++asid_cache & MM_CTXT_ASID_MASK))) { - /* move to new ASID */ - if (++asid_cache > MAX_ASID) { /* ASID roll-over */ - asid_cache = FIRST_ASID; flush_tlb_all(); - } - /* - * Is next ASID already owned by some-one else (we are stealing it). - * If so, let the orig owner be aware of this, so when it runs, it - * asks for a brand new ASID. This would only happen for a long-lived - * 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 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 - * 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. - */ - prev_owner = asid_mm_map[asid_cache]; - if (prev_owner) - prev_owner->context.asid = NO_ASID; + /* + * Above checke for rollover of 8 bit ASID in 32 bit container. + * If the container itself wrapped around, set it to a non zero + * "generation" to distinguish from no context + */ + if (!asid_cache) + asid_cache = MM_CTXT_FIRST_CYCLE; + } /* Assign new ASID to tsk */ - asid_mm_map[asid_cache] = mm; mm->context.asid = asid_cache; set_hw: - write_aux_reg(ARC_REG_PID, mm->context.asid | MMU_ENABLE); + write_aux_reg(ARC_REG_PID, hw_pid(mm) | MMU_ENABLE); local_irq_restore(flags); } @@ -138,7 +104,7 @@ set_hw: static inline int init_new_context(struct task_struct *tsk, struct mm_struct *mm) { - mm->context.asid = NO_ASID; + mm->context.asid = MM_CTXT_NO_ASID; return 0; } @@ -167,14 +133,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, static inline void destroy_context(struct mm_struct *mm) { - unsigned long flags; - - local_irq_save(flags); - - asid_mm_map[mm->context.asid] = NULL; - mm->context.asid = NO_ASID; - - local_irq_restore(flags); + mm->context.asid = MM_CTXT_NO_ASID; } /* it seemed that deactivate_mm( ) is a reasonable place to do book-keeping |