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 an 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, jump labels fall 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#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
49# define HAVE_JUMP_LABEL
50#endif
51
52#ifndef __ASSEMBLY__
53
54#include <linux/types.h>
55#include <linux/compiler.h>
56#include <linux/bug.h>
57
58extern bool static_key_initialized;
59
60#define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized,		      \
61				    "%s used before call to jump_label_init", \
62				    __func__)
63
64#ifdef HAVE_JUMP_LABEL
65
66struct static_key {
67	atomic_t enabled;
68/* Set lsb bit to 1 if branch is default true, 0 ot */
69	struct jump_entry *entries;
70#ifdef CONFIG_MODULES
71	struct static_key_mod *next;
72#endif
73};
74
75#else
76struct static_key {
77	atomic_t enabled;
78};
79#endif	/* HAVE_JUMP_LABEL */
80#endif /* __ASSEMBLY__ */
81
82#ifdef HAVE_JUMP_LABEL
83#include <asm/jump_label.h>
84#endif
85
86#ifndef __ASSEMBLY__
87
88enum jump_label_type {
89	JUMP_LABEL_DISABLE = 0,
90	JUMP_LABEL_ENABLE,
91};
92
93struct module;
94
95#include <linux/atomic.h>
96
97static inline int static_key_count(struct static_key *key)
98{
99	return atomic_read(&key->enabled);
100}
101
102#ifdef HAVE_JUMP_LABEL
103
104#define JUMP_LABEL_TYPE_FALSE_BRANCH	0UL
105#define JUMP_LABEL_TYPE_TRUE_BRANCH	1UL
106#define JUMP_LABEL_TYPE_MASK		1UL
107
108static
109inline struct jump_entry *jump_label_get_entries(struct static_key *key)
110{
111	return (struct jump_entry *)((unsigned long)key->entries
112						& ~JUMP_LABEL_TYPE_MASK);
113}
114
115static inline bool jump_label_get_branch_default(struct static_key *key)
116{
117	if (((unsigned long)key->entries & JUMP_LABEL_TYPE_MASK) ==
118	    JUMP_LABEL_TYPE_TRUE_BRANCH)
119		return true;
120	return false;
121}
122
123static __always_inline bool static_key_false(struct static_key *key)
124{
125	return arch_static_branch(key);
126}
127
128static __always_inline bool static_key_true(struct static_key *key)
129{
130	return !static_key_false(key);
131}
132
133extern struct jump_entry __start___jump_table[];
134extern struct jump_entry __stop___jump_table[];
135
136extern void jump_label_init(void);
137extern void jump_label_lock(void);
138extern void jump_label_unlock(void);
139extern void arch_jump_label_transform(struct jump_entry *entry,
140				      enum jump_label_type type);
141extern void arch_jump_label_transform_static(struct jump_entry *entry,
142					     enum jump_label_type type);
143extern int jump_label_text_reserved(void *start, void *end);
144extern void static_key_slow_inc(struct static_key *key);
145extern void static_key_slow_dec(struct static_key *key);
146extern void jump_label_apply_nops(struct module *mod);
147
148#define STATIC_KEY_INIT_TRUE ((struct static_key)		\
149	{ .enabled = ATOMIC_INIT(1),				\
150	  .entries = (void *)JUMP_LABEL_TYPE_TRUE_BRANCH })
151#define STATIC_KEY_INIT_FALSE ((struct static_key)		\
152	{ .enabled = ATOMIC_INIT(0),				\
153	  .entries = (void *)JUMP_LABEL_TYPE_FALSE_BRANCH })
154
155#else  /* !HAVE_JUMP_LABEL */
156
157static __always_inline void jump_label_init(void)
158{
159	static_key_initialized = true;
160}
161
162static __always_inline bool static_key_false(struct static_key *key)
163{
164	if (unlikely(static_key_count(key) > 0))
165		return true;
166	return false;
167}
168
169static __always_inline bool static_key_true(struct static_key *key)
170{
171	if (likely(static_key_count(key) > 0))
172		return true;
173	return false;
174}
175
176static inline void static_key_slow_inc(struct static_key *key)
177{
178	STATIC_KEY_CHECK_USE();
179	atomic_inc(&key->enabled);
180}
181
182static inline void static_key_slow_dec(struct static_key *key)
183{
184	STATIC_KEY_CHECK_USE();
185	atomic_dec(&key->enabled);
186}
187
188static inline int jump_label_text_reserved(void *start, void *end)
189{
190	return 0;
191}
192
193static inline void jump_label_lock(void) {}
194static inline void jump_label_unlock(void) {}
195
196static inline int jump_label_apply_nops(struct module *mod)
197{
198	return 0;
199}
200
201#define STATIC_KEY_INIT_TRUE ((struct static_key) \
202		{ .enabled = ATOMIC_INIT(1) })
203#define STATIC_KEY_INIT_FALSE ((struct static_key) \
204		{ .enabled = ATOMIC_INIT(0) })
205
206#endif	/* HAVE_JUMP_LABEL */
207
208#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
209#define jump_label_enabled static_key_enabled
210
211static inline bool static_key_enabled(struct static_key *key)
212{
213	return static_key_count(key) > 0;
214}
215
216#endif	/* _LINUX_JUMP_LABEL_H */
217
218#endif /* __ASSEMBLY__ */
219