include/linux/jump_label.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _LINUX_JUMP_LABEL_H
   3 #define _LINUX_JUMP_LABEL_H
   4
   5 /*
   6  * Jump label support
   7  *
   8  * Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
   9  * Copyright (C) 2011-2012 Red Hat, Inc., Peter Zijlstra
  10  *
  11  * DEPRECATED API:
  12  *
  13  * The use of 'struct static_key' directly, is now DEPRECATED. In addition
  14  * static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
  15  *
  16  * struct static_key false = STATIC_KEY_INIT_FALSE;
  17  * struct static_key true = STATIC_KEY_INIT_TRUE;
  18  * static_key_true()
  19  * static_key_false()
  20  *
  21  * The updated API replacements are:
  22  *
  23  * DEFINE_STATIC_KEY_TRUE(key);
  24  * DEFINE_STATIC_KEY_FALSE(key);
  25  * DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
  26  * DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count);
  27  * static_branch_likely()
  28  * static_branch_unlikely()
  29  *
  30  * Jump labels provide an interface to generate dynamic branches using
  31  * self-modifying code. Assuming toolchain and architecture support, if we
  32  * define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
  33  * an "if (static_branch_unlikely(&key))" statement is an unconditional branch
  34  * (which defaults to false - and the true block is placed out of line).
  35  * Similarly, we can define an initially true key via
  36  * "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
  37  * "if (static_branch_unlikely(&key))", in which case we will generate an
  38  * unconditional branch to the out-of-line true branch. Keys that are
  39  * initially true or false can be using in both static_branch_unlikely()
  40  * and static_branch_likely() statements.
  41  *
  42  * At runtime we can change the branch target by setting the key
  43  * to true via a call to static_branch_enable(), or false using
  44  * static_branch_disable(). If the direction of the branch is switched by
  45  * these calls then we run-time modify the branch target via a
  46  * no-op -> jump or jump -> no-op conversion. For example, for an
  47  * initially false key that is used in an "if (static_branch_unlikely(&key))"
  48  * statement, setting the key to true requires us to patch in a jump
  49  * to the out-of-line of true branch.
  50  *
  51  * In addition to static_branch_{enable,disable}, we can also reference count
  52  * the key or branch direction via static_branch_{inc,dec}. Thus,
  53  * static_branch_inc() can be thought of as a 'make more true' and
  54  * static_branch_dec() as a 'make more false'.
  55  *
  56  * Since this relies on modifying code, the branch modifying functions
  57  * must be considered absolute slow paths (machine wide synchronization etc.).
  58  * OTOH, since the affected branches are unconditional, their runtime overhead
  59  * will be absolutely minimal, esp. in the default (off) case where the total
  60  * effect is a single NOP of appropriate size. The on case will patch in a jump
  61  * to the out-of-line block.
  62  *
  63  * When the control is directly exposed to userspace, it is prudent to delay the
  64  * decrement to avoid high frequency code modifications which can (and do)
  65  * cause significant performance degradation. Struct static_key_deferred and
  66  * static_key_slow_dec_deferred() provide for this.
  67  *
  68  * Lacking toolchain and or architecture support, static keys fall back to a
  69  * simple conditional branch.
  70  *
  71  * Additional babbling in: Documentation/static-keys.txt
  72  */
  73
  74 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
  75 # define HAVE_JUMP_LABEL
  76 #endif
  77
  78 #ifndef __ASSEMBLY__
  79
  80 #include <linux/types.h>
  81 #include <linux/compiler.h>
  82
  83 extern bool static_key_initialized;
  84
  85 #define STATIC_KEY_CHECK_USE(key) WARN(!static_key_initialized,               \
  86                                     "%s(): static key '%pS' used before call to jump_label_init()", \
  87                                     __func__, (key))
  88
  89 #ifdef HAVE_JUMP_LABEL
  90
  91 struct static_key {
  92         atomic_t enabled;
  93 /*
  94  * Note:
  95  *   To make anonymous unions work with old compilers, the static
  96  *   initialization of them requires brackets. This creates a dependency
  97  *   on the order of the struct with the initializers. If any fields
  98  *   are added, STATIC_KEY_INIT_TRUE and STATIC_KEY_INIT_FALSE may need
  99  *   to be modified.
 100  *
 101  * bit 0 => 1 if key is initially true
 102  *          0 if initially false
 103  * bit 1 => 1 if points to struct static_key_mod
 104  *          0 if points to struct jump_entry
 105  */
 106         union {
 107                 unsigned long type;
 108                 struct jump_entry *entries;
 109                 struct static_key_mod *next;
 110         };
 111 };
 112
 113 #else
 114 struct static_key {
 115         atomic_t enabled;
 116 };
 117 #endif  /* HAVE_JUMP_LABEL */
 118 #endif /* __ASSEMBLY__ */
 119
 120 #ifdef HAVE_JUMP_LABEL
 121 #include <asm/jump_label.h>
 122 #endif
 123
 124 #ifndef __ASSEMBLY__
 125
 126 enum jump_label_type {
 127         JUMP_LABEL_NOP = 0,
 128         JUMP_LABEL_JMP,
 129 };
 130
 131 struct module;
 132
 133 #ifdef HAVE_JUMP_LABEL
 134
 135 #define JUMP_TYPE_FALSE         0UL
 136 #define JUMP_TYPE_TRUE          1UL
 137 #define JUMP_TYPE_LINKED        2UL
 138 #define JUMP_TYPE_MASK          3UL
 139
 140 static __always_inline bool static_key_false(struct static_key *key)
 141 {
 142         return arch_static_branch(key, false);
 143 }
 144
 145 static __always_inline bool static_key_true(struct static_key *key)
 146 {
 147         return !arch_static_branch(key, true);
 148 }
 149
 150 extern struct jump_entry __start___jump_table[];
 151 extern struct jump_entry __stop___jump_table[];
 152
 153 extern void jump_label_init(void);
 154 extern void jump_label_lock(void);
 155 extern void jump_label_unlock(void);
 156 extern void arch_jump_label_transform(struct jump_entry *entry,
 157                                       enum jump_label_type type);
 158 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 159                                              enum jump_label_type type);
 160 extern int jump_label_text_reserved(void *start, void *end);
 161 extern void static_key_slow_inc(struct static_key *key);
 162 extern void static_key_slow_dec(struct static_key *key);
 163 extern void jump_label_apply_nops(struct module *mod);
 164 extern int static_key_count(struct static_key *key);
 165 extern void static_key_enable(struct static_key *key);
 166 extern void static_key_disable(struct static_key *key);
 167 extern void static_key_enable_cpuslocked(struct static_key *key);
 168 extern void static_key_disable_cpuslocked(struct static_key *key);
 169
 170 /*
 171  * We should be using ATOMIC_INIT() for initializing .enabled, but
 172  * the inclusion of atomic.h is problematic for inclusion of jump_label.h
 173  * in 'low-level' headers. Thus, we are initializing .enabled with a
 174  * raw value, but have added a BUILD_BUG_ON() to catch any issues in
 175  * jump_label_init() see: kernel/jump_label.c.
 176  */
 177 #define STATIC_KEY_INIT_TRUE                                    \
 178         { .enabled = { 1 },                                     \
 179           { .entries = (void *)JUMP_TYPE_TRUE } }
 180 #define STATIC_KEY_INIT_FALSE                                   \
 181         { .enabled = { 0 },                                     \
 182           { .entries = (void *)JUMP_TYPE_FALSE } }
 183
 184 #else  /* !HAVE_JUMP_LABEL */
 185
 186 #include <linux/atomic.h>
 187 #include <linux/bug.h>
 188
 189 static inline int static_key_count(struct static_key *key)
 190 {
 191         return atomic_read(&key->enabled);
 192 }
 193
 194 static __always_inline void jump_label_init(void)
 195 {
 196         static_key_initialized = true;
 197 }
 198
 199 static __always_inline bool static_key_false(struct static_key *key)
 200 {
 201         if (unlikely(static_key_count(key) > 0))
 202                 return true;
 203         return false;
 204 }
 205
 206 static __always_inline bool static_key_true(struct static_key *key)
 207 {
 208         if (likely(static_key_count(key) > 0))
 209                 return true;
 210         return false;
 211 }
 212
 213 static inline void static_key_slow_inc(struct static_key *key)
 214 {
 215         STATIC_KEY_CHECK_USE(key);
 216         atomic_inc(&key->enabled);
 217 }
 218
 219 static inline void static_key_slow_dec(struct static_key *key)
 220 {
 221         STATIC_KEY_CHECK_USE(key);
 222         atomic_dec(&key->enabled);
 223 }
 224
 225 static inline int jump_label_text_reserved(void *start, void *end)
 226 {
 227         return 0;
 228 }
 229
 230 static inline void jump_label_lock(void) {}
 231 static inline void jump_label_unlock(void) {}
 232
 233 static inline int jump_label_apply_nops(struct module *mod)
 234 {
 235         return 0;
 236 }
 237
 238 static inline void static_key_enable(struct static_key *key)
 239 {
 240         STATIC_KEY_CHECK_USE(key);
 241
 242         if (atomic_read(&key->enabled) != 0) {
 243                 WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
 244                 return;
 245         }
 246         atomic_set(&key->enabled, 1);
 247 }
 248
 249 static inline void static_key_disable(struct static_key *key)
 250 {
 251         STATIC_KEY_CHECK_USE(key);
 252
 253         if (atomic_read(&key->enabled) != 1) {
 254                 WARN_ON_ONCE(atomic_read(&key->enabled) != 0);
 255                 return;
 256         }
 257         atomic_set(&key->enabled, 0);
 258 }
 259
 260 #define static_key_enable_cpuslocked(k)         static_key_enable((k))
 261 #define static_key_disable_cpuslocked(k)        static_key_disable((k))
 262
 263 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
 264 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
 265
 266 #endif  /* HAVE_JUMP_LABEL */
 267
 268 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 269 #define jump_label_enabled static_key_enabled
 270
 271 /* -------------------------------------------------------------------------- */
 272
 273 /*
 274  * Two type wrappers around static_key, such that we can use compile time
 275  * type differentiation to emit the right code.
 276  *
 277  * All the below code is macros in order to play type games.
 278  */
 279
 280 struct static_key_true {
 281         struct static_key key;
 282 };
 283
 284 struct static_key_false {
 285         struct static_key key;
 286 };
 287
 288 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
 289 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
 290
 291 #define DEFINE_STATIC_KEY_TRUE(name)    \
 292         struct static_key_true name = STATIC_KEY_TRUE_INIT
 293
 294 #define DECLARE_STATIC_KEY_TRUE(name)   \
 295         extern struct static_key_true name
 296
 297 #define DEFINE_STATIC_KEY_FALSE(name)   \
 298         struct static_key_false name = STATIC_KEY_FALSE_INIT
 299
 300 #define DECLARE_STATIC_KEY_FALSE(name)  \
 301         extern struct static_key_false name
 302
 303 #define DEFINE_STATIC_KEY_ARRAY_TRUE(name, count)               \
 304         struct static_key_true name[count] = {                  \
 305                 [0 ... (count) - 1] = STATIC_KEY_TRUE_INIT,     \
 306         }
 307
 308 #define DEFINE_STATIC_KEY_ARRAY_FALSE(name, count)              \
 309         struct static_key_false name[count] = {                 \
 310                 [0 ... (count) - 1] = STATIC_KEY_FALSE_INIT,    \
 311         }
 312
 313 extern bool ____wrong_branch_error(void);
 314
 315 #define static_key_enabled(x)                                                   \
 316 ({                                                                              \
 317         if (!__builtin_types_compatible_p(typeof(*x), struct static_key) &&     \
 318             !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
 319             !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 320                 ____wrong_branch_error();                                       \
 321         static_key_count((struct static_key *)x) > 0;                           \
 322 })
 323
 324 #ifdef HAVE_JUMP_LABEL
 325
 326 /*
 327  * Combine the right initial value (type) with the right branch order
 328  * to generate the desired result.
 329  *
 330  *
 331  * type\branch| likely (1)            | unlikely (0)
 332  * -----------+-----------------------+------------------
 333  *            |                       |
 334  *  true (1)  |    ...                |    ...
 335  *            |    NOP                |    JMP L
 336  *            |    <br-stmts>         | 1: ...
 337  *            | L: ...                |
 338  *            |                       |
 339  *            |                       | L: <br-stmts>
 340  *            |                       |    jmp 1b
 341  *            |                       |
 342  * -----------+-----------------------+------------------
 343  *            |                       |
 344  *  false (0) |    ...                |    ...
 345  *            |    JMP L              |    NOP
 346  *            |    <br-stmts>         | 1: ...
 347  *            | L: ...                |
 348  *            |                       |
 349  *            |                       | L: <br-stmts>
 350  *            |                       |    jmp 1b
 351  *            |                       |
 352  * -----------+-----------------------+------------------
 353  *
 354  * The initial value is encoded in the LSB of static_key::entries,
 355  * type: 0 = false, 1 = true.
 356  *
 357  * The branch type is encoded in the LSB of jump_entry::key,
 358  * branch: 0 = unlikely, 1 = likely.
 359  *
 360  * This gives the following logic table:
 361  *
 362  *      enabled type    branch    instuction
 363  * -----------------------------+-----------
 364  *      0       0       0       | NOP
 365  *      0       0       1       | JMP
 366  *      0       1       0       | NOP
 367  *      0       1       1       | JMP
 368  *
 369  *      1       0       0       | JMP
 370  *      1       0       1       | NOP
 371  *      1       1       0       | JMP
 372  *      1       1       1       | NOP
 373  *
 374  * Which gives the following functions:
 375  *
 376  *   dynamic: instruction = enabled ^ branch
 377  *   static:  instruction = type ^ branch
 378  *
 379  * See jump_label_type() / jump_label_init_type().
 380  */
 381
 382 #define static_branch_likely(x)                                                 \
 383 ({                                                                              \
 384         bool branch;                                                            \
 385         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 386                 branch = !arch_static_branch(&(x)->key, true);                  \
 387         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 388                 branch = !arch_static_branch_jump(&(x)->key, true);             \
 389         else                                                                    \
 390                 branch = ____wrong_branch_error();                              \
 391         branch;                                                                 \
 392 })
 393
 394 #define static_branch_unlikely(x)                                               \
 395 ({                                                                              \
 396         bool branch;                                                            \
 397         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 398                 branch = arch_static_branch_jump(&(x)->key, false);             \
 399         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 400                 branch = arch_static_branch(&(x)->key, false);                  \
 401         else                                                                    \
 402                 branch = ____wrong_branch_error();                              \
 403         branch;                                                                 \
 404 })
 405
 406 #else /* !HAVE_JUMP_LABEL */
 407
 408 #define static_branch_likely(x)         likely(static_key_enabled(&(x)->key))
 409 #define static_branch_unlikely(x)       unlikely(static_key_enabled(&(x)->key))
 410
 411 #endif /* HAVE_JUMP_LABEL */
 412
 413 /*
 414  * Advanced usage; refcount, branch is enabled when: count != 0
 415  */
 416
 417 #define static_branch_inc(x)            static_key_slow_inc(&(x)->key)
 418 #define static_branch_dec(x)            static_key_slow_dec(&(x)->key)
 419
 420 /*
 421  * Normal usage; boolean enable/disable.
 422  */
 423
 424 #define static_branch_enable(x)                 static_key_enable(&(x)->key)
 425 #define static_branch_disable(x)                static_key_disable(&(x)->key)
 426 #define static_branch_enable_cpuslocked(x)      static_key_enable_cpuslocked(&(x)->key)
 427 #define static_branch_disable_cpuslocked(x)     static_key_disable_cpuslocked(&(x)->key)
 428
 429 #endif /* __ASSEMBLY__ */
 430
 431 #endif  /* _LINUX_JUMP_LABEL_H */