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1#ifndef _LINUX_JUMP_LABEL_H
2#define _LINUX_JUMP_LABEL_H
3
4/*
5 * Jump label support
6 *
7 * Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
8 * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>
9 *
10 * Jump labels provide an interface to generate dynamic branches using
11 * self-modifying code. Assuming toolchain and architecture support the result
12 * of a "if (static_key_false(&key))" statement is a unconditional branch (which
13 * defaults to false - and the true block is placed out of line).
14 *
15 * However at runtime we can change the branch target using
16 * static_key_slow_{inc,dec}(). These function as a 'reference' count on the key
17 * object and for as long as there are references all branches referring to
18 * that particular key will point to the (out of line) true block.
19 *
20 * Since this relies on modifying code the static_key_slow_{inc,dec}() functions
21 * must be considered absolute slow paths (machine wide synchronization etc.).
22 * OTOH, since the affected branches are unconditional their runtime overhead
23 * will be absolutely minimal, esp. in the default (off) case where the total
24 * effect is a single NOP of appropriate size. The on case will patch in a jump
25 * to the out-of-line block.
26 *
27 * When the control is directly exposed to userspace it is prudent to delay the
28 * decrement to avoid high frequency code modifications which can (and do)
29 * cause significant performance degradation. Struct static_key_deferred and
30 * static_key_slow_dec_deferred() provide for this.
31 *
32 * Lacking toolchain and or architecture support, it falls back to a simple
33 * conditional branch.
34 *
35 * struct static_key my_key = STATIC_KEY_INIT_TRUE;
36 *
37 * if (static_key_true(&my_key)) {
38 * }
39 *
40 * will result in the true case being in-line and starts the key with a single
41 * reference. Mixing static_key_true() and static_key_false() on the same key is not
42 * allowed.
43 *
44 * Not initializing the key (static data is initialized to 0s anyway) is the
45 * same as using STATIC_KEY_INIT_FALSE.
46 *
47*/
48
49#include <linux/types.h>
50#include <linux/compiler.h>
51#include <linux/bug.h>
52
53extern bool static_key_initialized;
54
55#define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized, \
56 "%s used before call to jump_label_init", \
57 __func__)
58
59#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
60
61struct static_key {
62 atomic_t enabled;
63/* Set lsb bit to 1 if branch is default true, 0 ot */
64 struct jump_entry *entries;
65#ifdef CONFIG_MODULES
66 struct static_key_mod *next;
67#endif
68};
69
70# include <asm/jump_label.h>
71# define HAVE_JUMP_LABEL
72#endif /* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */
73
74enum jump_label_type {
75 JUMP_LABEL_DISABLE = 0,
76 JUMP_LABEL_ENABLE,
77};
78
79struct module;
80
81#include <linux/atomic.h>
82#ifdef HAVE_JUMP_LABEL
83
84#define JUMP_LABEL_TRUE_BRANCH 1UL
85
86static
87inline struct jump_entry *jump_label_get_entries(struct static_key *key)
88{
89 return (struct jump_entry *)((unsigned long)key->entries
90 & ~JUMP_LABEL_TRUE_BRANCH);
91}
92
93static inline bool jump_label_get_branch_default(struct static_key *key)
94{
95 if ((unsigned long)key->entries & JUMP_LABEL_TRUE_BRANCH)
96 return true;
97 return false;
98}
99
100static __always_inline bool static_key_false(struct static_key *key)
101{
102 return arch_static_branch(key);
103}
104
105static __always_inline bool static_key_true(struct static_key *key)
106{
107 return !static_key_false(key);
108}
109
110extern struct jump_entry __start___jump_table[];
111extern struct jump_entry __stop___jump_table[];
112
113extern void jump_label_init(void);
114extern void jump_label_lock(void);
115extern void jump_label_unlock(void);
116extern void arch_jump_label_transform(struct jump_entry *entry,
117 enum jump_label_type type);
118extern void arch_jump_label_transform_static(struct jump_entry *entry,
119 enum jump_label_type type);
120extern int jump_label_text_reserved(void *start, void *end);
121extern void static_key_slow_inc(struct static_key *key);
122extern void static_key_slow_dec(struct static_key *key);
123extern void jump_label_apply_nops(struct module *mod);
124
125#define STATIC_KEY_INIT_TRUE ((struct static_key) \
126 { .enabled = ATOMIC_INIT(1), .entries = (void *)1 })
127#define STATIC_KEY_INIT_FALSE ((struct static_key) \
128 { .enabled = ATOMIC_INIT(0), .entries = (void *)0 })
129
130#else /* !HAVE_JUMP_LABEL */
131
132struct static_key {
133 atomic_t enabled;
134};
135
136static __always_inline void jump_label_init(void)
137{
138 static_key_initialized = true;
139}
140
141static __always_inline bool static_key_false(struct static_key *key)
142{
143 if (unlikely(atomic_read(&key->enabled) > 0))
144 return true;
145 return false;
146}
147
148static __always_inline bool static_key_true(struct static_key *key)
149{
150 if (likely(atomic_read(&key->enabled) > 0))
151 return true;
152 return false;
153}
154
155static inline void static_key_slow_inc(struct static_key *key)
156{
157 STATIC_KEY_CHECK_USE();
158 atomic_inc(&key->enabled);
159}
160
161static inline void static_key_slow_dec(struct static_key *key)
162{
163 STATIC_KEY_CHECK_USE();
164 atomic_dec(&key->enabled);
165}
166
167static inline int jump_label_text_reserved(void *start, void *end)
168{
169 return 0;
170}
171
172static inline void jump_label_lock(void) {}
173static inline void jump_label_unlock(void) {}
174
175static inline int jump_label_apply_nops(struct module *mod)
176{
177 return 0;
178}
179
180#define STATIC_KEY_INIT_TRUE ((struct static_key) \
181 { .enabled = ATOMIC_INIT(1) })
182#define STATIC_KEY_INIT_FALSE ((struct static_key) \
183 { .enabled = ATOMIC_INIT(0) })
184
185#endif /* HAVE_JUMP_LABEL */
186
187#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
188#define jump_label_enabled static_key_enabled
189
190static inline bool static_key_enabled(struct static_key *key)
191{
192 return (atomic_read(&key->enabled) > 0);
193}
194
195#endif /* _LINUX_JUMP_LABEL_H */