include/linux/jump_label.h

   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 Red Hat, Inc., Peter Zijlstra
   9  *
  10  * DEPRECATED API:
  11  *
  12  * The use of 'struct static_key' directly, is now DEPRECATED. In addition
  13  * static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
  14  *
  15  * struct static_key false = STATIC_KEY_INIT_FALSE;
  16  * struct static_key true = STATIC_KEY_INIT_TRUE;
  17  * static_key_true()
  18  * static_key_false()
  19  *
  20  * The updated API replacements are:
  21  *
  22  * DEFINE_STATIC_KEY_TRUE(key);
  23  * DEFINE_STATIC_KEY_FALSE(key);
  24  * static_branch_likely()
  25  * static_branch_unlikely()
  26  *
  27  * Jump labels provide an interface to generate dynamic branches using
  28  * self-modifying code. Assuming toolchain and architecture support, if we
  29  * define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
  30  * an "if (static_branch_unlikely(&key))" statement is an unconditional branch
  31  * (which defaults to false - and the true block is placed out of line).
  32  * Similarly, we can define an initially true key via
  33  * "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
  34  * "if (static_branch_unlikely(&key))", in which case we will generate an
  35  * unconditional branch to the out-of-line true branch. Keys that are
  36  * initially true or false can be using in both static_branch_unlikely()
  37  * and static_branch_likely() statements.
  38  *
  39  * At runtime we can change the branch target by setting the key
  40  * to true via a call to static_branch_enable(), or false using
  41  * static_branch_disable(). If the direction of the branch is switched by
  42  * these calls then we run-time modify the branch target via a
  43  * no-op -> jump or jump -> no-op conversion. For example, for an
  44  * initially false key that is used in an "if (static_branch_unlikely(&key))"
  45  * statement, setting the key to true requires us to patch in a jump
  46  * to the out-of-line of true branch.
  47  *
  48  * In addition to static_branch_{enable,disable}, we can also reference count
  49  * the key or branch direction via static_branch_{inc,dec}. Thus,
  50  * static_branch_inc() can be thought of as a 'make more true' and
  51  * static_branch_dec() as a 'make more false'.
  52  *
  53  * Since this relies on modifying code, the branch modifying functions
  54  * must be considered absolute slow paths (machine wide synchronization etc.).
  55  * OTOH, since the affected branches are unconditional, their runtime overhead
  56  * will be absolutely minimal, esp. in the default (off) case where the total
  57  * effect is a single NOP of appropriate size. The on case will patch in a jump
  58  * to the out-of-line block.
  59  *
  60  * When the control is directly exposed to userspace, it is prudent to delay the
  61  * decrement to avoid high frequency code modifications which can (and do)
  62  * cause significant performance degradation. Struct static_key_deferred and
  63  * static_key_slow_dec_deferred() provide for this.
  64  *
  65  * Lacking toolchain and or architecture support, static keys fall back to a
  66  * simple conditional branch.
  67  *
  68  * Additional babbling in: Documentation/static-keys.txt
  69  */
  70
  71 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  72 # define HAVE_JUMP_LABEL
  73 #endif
  74
  75 #ifndef __ASSEMBLY__
  76
  77 #include <linux/types.h>
  78 #include <linux/compiler.h>
  79 #include <linux/bug.h>
  80
  81 extern bool static_key_initialized;
  82
  83 #define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized,                  \
  84                                     "%s used before call to jump_label_init", \
  85                                     __func__)
  86
  87 #ifdef HAVE_JUMP_LABEL
  88
  89 struct static_key {
  90         atomic_t enabled;
  91 /* Set lsb bit to 1 if branch is default true, 0 ot */
  92         struct jump_entry *entries;
  93 #ifdef CONFIG_MODULES
  94         struct static_key_mod *next;
  95 #endif
  96 };
  97
  98 #else
  99 struct static_key {
 100         atomic_t enabled;
 101 };
 102 #endif  /* HAVE_JUMP_LABEL */
 103 #endif /* __ASSEMBLY__ */
 104
 105 #ifdef HAVE_JUMP_LABEL
 106 #include <asm/jump_label.h>
 107 #endif
 108
 109 #ifndef __ASSEMBLY__
 110
 111 enum jump_label_type {
 112         JUMP_LABEL_NOP = 0,
 113         JUMP_LABEL_JMP,
 114 };
 115
 116 struct module;
 117
 118 #include <linux/atomic.h>
 119
 120 static inline int static_key_count(struct static_key *key)
 121 {
 122         return atomic_read(&key->enabled);
 123 }
 124
 125 #ifdef HAVE_JUMP_LABEL
 126
 127 #define JUMP_TYPE_FALSE 0UL
 128 #define JUMP_TYPE_TRUE  1UL
 129 #define JUMP_TYPE_MASK  1UL
 130
 131 static __always_inline bool static_key_false(struct static_key *key)
 132 {
 133         return arch_static_branch(key, false);
 134 }
 135
 136 static __always_inline bool static_key_true(struct static_key *key)
 137 {
 138         return !arch_static_branch(key, true);
 139 }
 140
 141 extern struct jump_entry __start___jump_table[];
 142 extern struct jump_entry __stop___jump_table[];
 143
 144 extern void jump_label_init(void);
 145 extern void jump_label_lock(void);
 146 extern void jump_label_unlock(void);
 147 extern void arch_jump_label_transform(struct jump_entry *entry,
 148                                       enum jump_label_type type);
 149 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 150                                              enum jump_label_type type);
 151 extern int jump_label_text_reserved(void *start, void *end);
 152 extern void static_key_slow_inc(struct static_key *key);
 153 extern void static_key_slow_dec(struct static_key *key);
 154 extern void jump_label_apply_nops(struct module *mod);
 155
 156 #define STATIC_KEY_INIT_TRUE                                    \
 157         { .enabled = ATOMIC_INIT(1),                            \
 158           .entries = (void *)JUMP_TYPE_TRUE }
 159 #define STATIC_KEY_INIT_FALSE                                   \
 160         { .enabled = ATOMIC_INIT(0),                            \
 161           .entries = (void *)JUMP_TYPE_FALSE }
 162
 163 #else  /* !HAVE_JUMP_LABEL */
 164
 165 static __always_inline void jump_label_init(void)
 166 {
 167         static_key_initialized = true;
 168 }
 169
 170 static __always_inline bool static_key_false(struct static_key *key)
 171 {
 172         if (unlikely(static_key_count(key) > 0))
 173                 return true;
 174         return false;
 175 }
 176
 177 static __always_inline bool static_key_true(struct static_key *key)
 178 {
 179         if (likely(static_key_count(key) > 0))
 180                 return true;
 181         return false;
 182 }
 183
 184 static inline void static_key_slow_inc(struct static_key *key)
 185 {
 186         STATIC_KEY_CHECK_USE();
 187         atomic_inc(&key->enabled);
 188 }
 189
 190 static inline void static_key_slow_dec(struct static_key *key)
 191 {
 192         STATIC_KEY_CHECK_USE();
 193         atomic_dec(&key->enabled);
 194 }
 195
 196 static inline int jump_label_text_reserved(void *start, void *end)
 197 {
 198         return 0;
 199 }
 200
 201 static inline void jump_label_lock(void) {}
 202 static inline void jump_label_unlock(void) {}
 203
 204 static inline int jump_label_apply_nops(struct module *mod)
 205 {
 206         return 0;
 207 }
 208
 209 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
 210 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
 211
 212 #endif  /* HAVE_JUMP_LABEL */
 213
 214 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 215 #define jump_label_enabled static_key_enabled
 216
 217 static inline void static_key_enable(struct static_key *key)
 218 {
 219         int count = static_key_count(key);
 220
 221         WARN_ON_ONCE(count < 0 || count > 1);
 222
 223         if (!count)
 224                 static_key_slow_inc(key);
 225 }
 226
 227 static inline void static_key_disable(struct static_key *key)
 228 {
 229         int count = static_key_count(key);
 230
 231         WARN_ON_ONCE(count < 0 || count > 1);
 232
 233         if (count)
 234                 static_key_slow_dec(key);
 235 }
 236
 237 /* -------------------------------------------------------------------------- */
 238
 239 /*
 240  * Two type wrappers around static_key, such that we can use compile time
 241  * type differentiation to emit the right code.
 242  *
 243  * All the below code is macros in order to play type games.
 244  */
 245
 246 struct static_key_true {
 247         struct static_key key;
 248 };
 249
 250 struct static_key_false {
 251         struct static_key key;
 252 };
 253
 254 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
 255 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
 256
 257 #define DEFINE_STATIC_KEY_TRUE(name)    \
 258         struct static_key_true name = STATIC_KEY_TRUE_INIT
 259
 260 #define DEFINE_STATIC_KEY_FALSE(name)   \
 261         struct static_key_false name = STATIC_KEY_FALSE_INIT
 262
 263 extern bool ____wrong_branch_error(void);
 264
 265 #define static_key_enabled(x)                                                   \
 266 ({                                                                              \
 267         if (!__builtin_types_compatible_p(typeof(*x), struct static_key) &&     \
 268             !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
 269             !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 270                 ____wrong_branch_error();                                       \
 271         static_key_count((struct static_key *)x) > 0;                           \
 272 })
 273
 274 #ifdef HAVE_JUMP_LABEL
 275
 276 /*
 277  * Combine the right initial value (type) with the right branch order
 278  * to generate the desired result.
 279  *
 280  *
 281  * type\branch| likely (1)            | unlikely (0)
 282  * -----------+-----------------------+------------------
 283  *            |                       |
 284  *  true (1)  |    ...                |    ...
 285  *            |    NOP                |    JMP L
 286  *            |    <br-stmts>         | 1: ...
 287  *            | L: ...                |
 288  *            |                       |
 289  *            |                       | L: <br-stmts>
 290  *            |                       |    jmp 1b
 291  *            |                       |
 292  * -----------+-----------------------+------------------
 293  *            |                       |
 294  *  false (0) |    ...                |    ...
 295  *            |    JMP L              |    NOP
 296  *            |    <br-stmts>         | 1: ...
 297  *            | L: ...                |
 298  *            |                       |
 299  *            |                       | L: <br-stmts>
 300  *            |                       |    jmp 1b
 301  *            |                       |
 302  * -----------+-----------------------+------------------
 303  *
 304  * The initial value is encoded in the LSB of static_key::entries,
 305  * type: 0 = false, 1 = true.
 306  *
 307  * The branch type is encoded in the LSB of jump_entry::key,
 308  * branch: 0 = unlikely, 1 = likely.
 309  *
 310  * This gives the following logic table:
 311  *
 312  *      enabled type    branch    instuction
 313  * -----------------------------+-----------
 314  *      0       0       0       | NOP
 315  *      0       0       1       | JMP
 316  *      0       1       0       | NOP
 317  *      0       1       1       | JMP
 318  *
 319  *      1       0       0       | JMP
 320  *      1       0       1       | NOP
 321  *      1       1       0       | JMP
 322  *      1       1       1       | NOP
 323  *
 324  * Which gives the following functions:
 325  *
 326  *   dynamic: instruction = enabled ^ branch
 327  *   static:  instruction = type ^ branch
 328  *
 329  * See jump_label_type() / jump_label_init_type().
 330  */
 331
 332 #define static_branch_likely(x)                                                 \
 333 ({                                                                              \
 334         bool branch;                                                            \
 335         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 336                 branch = !arch_static_branch(&(x)->key, true);                  \
 337         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 338                 branch = !arch_static_branch_jump(&(x)->key, true);             \
 339         else                                                                    \
 340                 branch = ____wrong_branch_error();                              \
 341         branch;                                                                 \
 342 })
 343
 344 #define static_branch_unlikely(x)                                               \
 345 ({                                                                              \
 346         bool branch;                                                            \
 347         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 348                 branch = arch_static_branch_jump(&(x)->key, false);             \
 349         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 350                 branch = arch_static_branch(&(x)->key, false);                  \
 351         else                                                                    \
 352                 branch = ____wrong_branch_error();                              \
 353         branch;                                                                 \
 354 })
 355
 356 #else /* !HAVE_JUMP_LABEL */
 357
 358 #define static_branch_likely(x)         likely(static_key_enabled(&(x)->key))
 359 #define static_branch_unlikely(x)       unlikely(static_key_enabled(&(x)->key))
 360
 361 #endif /* HAVE_JUMP_LABEL */
 362
 363 /*
 364  * Advanced usage; refcount, branch is enabled when: count != 0
 365  */
 366
 367 #define static_branch_inc(x)            static_key_slow_inc(&(x)->key)
 368 #define static_branch_dec(x)            static_key_slow_dec(&(x)->key)
 369
 370 /*
 371  * Normal usage; boolean enable/disable.
 372  */
 373
 374 #define static_branch_enable(x)         static_key_enable(&(x)->key)
 375 #define static_branch_disable(x)        static_key_disable(&(x)->key)
 376
 377 #endif  /* _LINUX_JUMP_LABEL_H */
 378
 379 #endif /* __ASSEMBLY__ */