diff options
Diffstat (limited to 'arch/arc/include/asm/mmu_context.h')
| -rw-r--r-- | arch/arc/include/asm/mmu_context.h | 161 |
1 files changed, 49 insertions, 112 deletions
diff --git a/arch/arc/include/asm/mmu_context.h b/arch/arc/include/asm/mmu_context.h index 0d71fb1..43a1b51 100644 --- a/arch/arc/include/asm/mmu_context.h +++ b/arch/arc/include/asm/mmu_context.h @@ -34,95 +34,65 @@ * 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; /* - * 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) { - struct mm_struct *prev_owner; unsigned long flags; local_irq_save(flags); /* - * Relinquish the currently owned ASID (if any). - * Doing unconditionally saves a cmp-n-branch; for already unused - * ASID slot, the value was/remains NULL + * 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. */ - asid_mm_map[mm->context.asid] = (struct mm_struct *)NULL; + if (!((mm->context.asid ^ asid_cache) & MM_CTXT_CYCLE_MASK)) + goto set_hw; + + /* 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 switch_mm( ) 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 - * 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; -#ifdef CONFIG_ARC_TLB_DBG - pr_info("ARC_TLB_DBG: NewMM=0x%x OldMM=0x%x task_struct=0x%x Task: %s," - " pid:%u, assigned asid:%lu\n", - (unsigned int)mm, (unsigned int)prev_owner, - (unsigned int)(mm->context.tsk), (mm->context.tsk)->comm, - (mm->context.tsk)->pid, mm->context.asid); -#endif - - write_aux_reg(ARC_REG_PID, asid_cache | MMU_ENABLE); +set_hw: + write_aux_reg(ARC_REG_PID, hw_pid(mm) | MMU_ENABLE); local_irq_restore(flags); } @@ -134,10 +104,7 @@ static inline void get_new_mmu_context(struct mm_struct *mm) static inline int init_new_context(struct task_struct *tsk, struct mm_struct *mm) { - mm->context.asid = NO_ASID; -#ifdef CONFIG_ARC_TLB_DBG - mm->context.tsk = tsk; -#endif + mm->context.asid = MM_CTXT_NO_ASID; return 0; } @@ -152,40 +119,21 @@ 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); } +/* + * Called at the time of execve() to get a new ASID + * Note the subtlety here: get_new_mmu_context() behaves differently here + * vs. in switch_mm(). Here it always returns a new ASID, because mm has + * an unallocated "initial" value, while in latter, it moves to a new ASID, + * only if it was unallocated + */ +#define activate_mm(prev, next) switch_mm(prev, next, NULL) + 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 @@ -197,17 +145,6 @@ static inline void destroy_context(struct mm_struct *mm) */ #define deactivate_mm(tsk, mm) do { } while (0) -static inline void activate_mm(struct mm_struct *prev, struct mm_struct *next) -{ -#ifndef CONFIG_SMP - write_aux_reg(ARC_REG_SCRATCH_DATA0, next->pgd); -#endif - - /* Unconditionally get a new ASID */ - get_new_mmu_context(next); - -} - #define enter_lazy_tlb(mm, tsk) #endif /* __ASM_ARC_MMU_CONTEXT_H */ |
