1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3  * This file contains the routines for handling the MMU on those
4  * PowerPC implementations where the MMU substantially follows the
5  * architecture specification.  This includes the 6xx, 7xx, 7xxx,
6  * and 8260 implementations but excludes the 8xx and 4xx.
7  *  -- paulus
8  *
9  *  Derived from arch/ppc/mm/init.c:
10  *    Copyright (C) 1995-1996 Gary Thomas (gdt@linuxppc.org)
11  *
12  *  Modifications by Paul Mackerras (PowerMac) (paulus@cs.anu.edu.au)
13  *  and Cort Dougan (PReP) (cort@cs.nmt.edu)
14  *    Copyright (C) 1996 Paul Mackerras
15  *
16  *  Derived from "arch/i386/mm/init.c"
17  *    Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
18  */
19 
20 #include <linux/mm.h>
21 #include <linux/init.h>
22 #include <linux/export.h>
23 
24 #include <asm/mmu_context.h>
25 
26 /*
27  * On 32-bit PowerPC 6xx/7xx/7xxx CPUs, we use a set of 16 VSIDs
28  * (virtual segment identifiers) for each context.  Although the
29  * hardware supports 24-bit VSIDs, and thus >1 million contexts,
30  * we only use 32,768 of them.  That is ample, since there can be
31  * at most around 30,000 tasks in the system anyway, and it means
32  * that we can use a bitmap to indicate which contexts are in use.
33  * Using a bitmap means that we entirely avoid all of the problems
34  * that we used to have when the context number overflowed,
35  * particularly on SMP systems.
36  *  -- paulus.
37  */
38 #define NO_CONTEXT      	((unsigned long) -1)
39 #define LAST_CONTEXT    	32767
40 #define FIRST_CONTEXT    	1
41 
42 /*
43  * This function defines the mapping from contexts to VSIDs (virtual
44  * segment IDs).  We use a skew on both the context and the high 4 bits
45  * of the 32-bit virtual address (the "effective segment ID") in order
46  * to spread out the entries in the MMU hash table.  Note, if this
47  * function is changed then arch/ppc/mm/hashtable.S will have to be
48  * changed to correspond.
49  *
50  *
51  * CTX_TO_VSID(ctx, va)	(((ctx) * (897 * 16) + ((va) >> 28) * 0x111) \
52  *				 & 0xffffff)
53  */
54 
55 static unsigned long next_mmu_context;
56 static unsigned long context_map[LAST_CONTEXT / BITS_PER_LONG + 1];
57 
58 unsigned long __init_new_context(void)
59 {
60 	unsigned long ctx = next_mmu_context;
61 
62 	while (test_and_set_bit(ctx, context_map)) {
63 		ctx = find_next_zero_bit(context_map, LAST_CONTEXT+1, ctx);
64 		if (ctx > LAST_CONTEXT)
65 			ctx = 0;
66 	}
67 	next_mmu_context = (ctx + 1) & LAST_CONTEXT;
68 
69 	return ctx;
70 }
71 EXPORT_SYMBOL_GPL(__init_new_context);
72 
73 /*
74  * Set up the context for a new address space.
75  */
76 int init_new_context(struct task_struct *t, struct mm_struct *mm)
77 {
78 	mm->context.id = __init_new_context();
79 
80 	return 0;
81 }
82 
83 /*
84  * Free a context ID. Make sure to call this with preempt disabled!
85  */
86 void __destroy_context(unsigned long ctx)
87 {
88 	clear_bit(ctx, context_map);
89 }
90 EXPORT_SYMBOL_GPL(__destroy_context);
91 
92 /*
93  * We're finished using the context for an address space.
94  */
95 void destroy_context(struct mm_struct *mm)
96 {
97 	preempt_disable();
98 	if (mm->context.id != NO_CONTEXT) {
99 		__destroy_context(mm->context.id);
100 		mm->context.id = NO_CONTEXT;
101 	}
102 	preempt_enable();
103 }
104 
105 /*
106  * Initialize the context management stuff.
107  */
108 void __init mmu_context_init(void)
109 {
110 	/* Reserve context 0 for kernel use */
111 	context_map[0] = (1 << FIRST_CONTEXT) - 1;
112 	next_mmu_context = FIRST_CONTEXT;
113 }
114