mmu_context.h

《嵌入式系统设计与实例开发实验教材二源码》Linux内核移植与编译实验

/*
 * BK Id: SCCS/s.mmu_context.h 1.18 09/26/01 16:02:49 paulus
 */
#ifdef __KERNEL__
#ifndef __PPC_MMU_CONTEXT_H
#define __PPC_MMU_CONTEXT_H

#include <linux/config.h>
#include <asm/atomic.h>
#include <asm/bitops.h>
#include <asm/mmu.h>

/*
 * On 32-bit PowerPC 6xx/7xx/7xxx CPUs, we use a set of 16 VSIDs
 * (virtual segment identifiers) for each context.  Although the
 * hardware supports 24-bit VSIDs, and thus >1 million contexts,
 * we only use 32,768 of them.  That is ample, since there can be
 * at most around 30,000 tasks in the system anyway, and it means
 * that we can use a bitmap to indicate which contexts are in use.
 * Using a bitmap means that we entirely avoid all of the problems
 * that we used to have when the context number overflowed,
 * particularly on SMP systems.
 *  -- paulus.
 */

/*
 * This function defines the mapping from contexts to VSIDs (virtual
 * segment IDs).  We use a skew on both the context and the high 4 bits
 * of the 32-bit virtual address (the "effective segment ID") in order
 * to spread out the entries in the MMU hash table.  Note, if this
 * function is changed then arch/ppc/mm/hashtable.S will have to be
 * changed to correspond.
 */
#define CTX_TO_VSID(ctx, va)	(((ctx) * (897 * 16) + ((va) >> 28) * 0x111) \
				 & 0xffffff)

/*
   The MPC8xx has only 16 contexts.  We rotate through them on each
   task switch.  A better way would be to keep track of tasks that
   own contexts, and implement an LRU usage.  That way very active
   tasks don't always have to pay the TLB reload overhead.  The
   kernel pages are mapped shared, so the kernel can run on behalf
   of any task that makes a kernel entry.  Shared does not mean they
   are not protected, just that the ASID comparison is not performed.
        -- Dan

   The IBM4xx has 256 contexts, so we can just rotate through these
   as a way of "switching" contexts.  If the TID of the TLB is zero,
   the PID/TID comparison is disabled, so we can use a TID of zero
   to represent all kernel pages as shared among all contexts.
   	-- Dan
 */

static inline void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk, unsigned cpu)
{
}

#ifdef CONFIG_8xx
#define NO_CONTEXT      	16
#define LAST_CONTEXT    	15
#define FIRST_CONTEXT    	0

#elif CONFIG_4xx
#define NO_CONTEXT      	256
#define LAST_CONTEXT    	255
#define FIRST_CONTEXT    	1

#else

/* PPC 6xx, 7xx CPUs */
#define NO_CONTEXT      	((mm_context_t) -1)
#define LAST_CONTEXT    	32767
#define FIRST_CONTEXT    	1
#endif

/*
 * Set the current MMU context.
 * On 32-bit PowerPCs (other than the 8xx embedded chips), this is done by
 * loading up the segment registers for the user part of the address space.
 *
 * Since the PGD is immediately available, it is much faster to simply
 * pass this along as a second parameter, which is required for 8xx and
 * can be used for debugging on all processors (if you happen to have
 * an Abatron).
 */
extern void set_context(mm_context_t context, pgd_t *pgd);

/*
 * Bitmap of contexts in use.
 * The size of this bitmap is LAST_CONTEXT + 1 bits.
 */
extern unsigned long context_map[];

/*
 * This caches the next context number that we expect to be free.
 * Its use is an optimization only, we can't rely on this context
 * number to be free, but it usually will be.
 */
extern mm_context_t next_mmu_context;

/*
 * If we don't have sufficient contexts to give one to every task
 * that could be in the system, we need to be able to steal contexts.
 * These variables support that.
 */
#if LAST_CONTEXT < 30000
#define FEW_CONTEXTS	1
extern atomic_t nr_free_contexts;
extern struct mm_struct *context_mm[LAST_CONTEXT+1];
extern void steal_context(void);
#endif

/*
 * Get a new mmu context for the address space described by `mm'.
 */
static inline void get_mmu_context(struct mm_struct *mm)
{
	mm_context_t ctx;

	if (mm->context != NO_CONTEXT)
		return;
#ifdef FEW_CONTEXTS
	while (atomic_dec_if_positive(&nr_free_contexts) < 0)
		steal_context();
#endif
	ctx = next_mmu_context;
	while (test_and_set_bit(ctx, context_map)) {
		ctx = find_next_zero_bit(context_map, LAST_CONTEXT+1, ctx);
		if (ctx > LAST_CONTEXT)
			ctx = 0;
	}
	next_mmu_context = (ctx + 1) & LAST_CONTEXT;
	mm->context = ctx;
#ifdef FEW_CONTEXTS
	context_mm[ctx] = mm;
#endif
}

/*
 * Set up the context for a new address space.
 */
#define init_new_context(tsk,mm)	(((mm)->context = NO_CONTEXT), 0)

/*
 * We're finished using the context for an address space.
 */
static inline void destroy_context(struct mm_struct *mm)
{
	if (mm->context != NO_CONTEXT) {
		clear_bit(mm->context, context_map);
		mm->context = NO_CONTEXT;
#ifdef FEW_CONTEXTS
		atomic_inc(&nr_free_contexts);
#endif
	}
}

static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
			     struct task_struct *tsk, int cpu)
{
	tsk->thread.pgdir = next->pgd;
	get_mmu_context(next);
	set_context(next->context, next->pgd);
}

/*
 * After we have set current->mm to a new value, this activates
 * the context for the new mm so we see the new mappings.
 */
static inline void activate_mm(struct mm_struct *active_mm, struct mm_struct *mm)
{
	current->thread.pgdir = mm->pgd;
	get_mmu_context(mm);
	set_context(mm->context, mm->pgd);
}

extern void mmu_context_init(void);

#endif /* __PPC_MMU_CONTEXT_H */
#endif /* __KERNEL__ */