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