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
 123 #ifndef __ASSEMBLY__
 124 #ifdef CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE
 125
 126 struct jump_entry {
 127         s32 code;
 128         s32 target;
 129         long key;       // key may be far away from the core kernel under KASLR
 130 };
 131
 132 static inline unsigned long jump_entry_code(const struct jump_entry *entry)
 133 {
 134         return (unsigned long)&entry->code + entry->code;
 135 }
 136
 137 static inline unsigned long jump_entry_target(const struct jump_entry *entry)
 138 {
 139         return (unsigned long)&entry->target + entry->target;
 140 }
 141
 142 static inline struct static_key *jump_entry_key(const struct jump_entry *entry)
 143 {
 144         long offset = entry->key & ~3L;
 145
 146         return (struct static_key *)((unsigned long)&entry->key + offset);
 147 }
 148
 149 #else
 150
 151 static inline unsigned long jump_entry_code(const struct jump_entry *entry)
 152 {
 153         return entry->code;
 154 }
 155
 156 static inline unsigned long jump_entry_target(const struct jump_entry *entry)
 157 {
 158         return entry->target;
 159 }
 160
 161 static inline struct static_key *jump_entry_key(const struct jump_entry *entry)
 162 {
 163         return (struct static_key *)((unsigned long)entry->key & ~3UL);
 164 }
 165
 166 #endif
 167
 168 static inline bool jump_entry_is_branch(const struct jump_entry *entry)
 169 {
 170         return (unsigned long)entry->key & 1UL;
 171 }
 172
 173 static inline bool jump_entry_is_init(const struct jump_entry *entry)
 174 {
 175         return (unsigned long)entry->key & 2UL;
 176 }
 177
 178 static inline void jump_entry_set_init(struct jump_entry *entry)
 179 {
 180         entry->key |= 2;
 181 }
 182
 183 #endif
 184 #endif
 185
 186 #ifndef __ASSEMBLY__
 187
 188 enum jump_label_type {
 189         JUMP_LABEL_NOP = 0,
 190         JUMP_LABEL_JMP,
 191 };
 192
 193 struct module;
 194
 195 #ifdef HAVE_JUMP_LABEL
 196
 197 #define JUMP_TYPE_FALSE         0UL
 198 #define JUMP_TYPE_TRUE          1UL
 199 #define JUMP_TYPE_LINKED        2UL
 200 #define JUMP_TYPE_MASK          3UL
 201
 202 static __always_inline bool static_key_false(struct static_key *key)
 203 {
 204         return arch_static_branch(key, false);
 205 }
 206
 207 static __always_inline bool static_key_true(struct static_key *key)
 208 {
 209         return !arch_static_branch(key, true);
 210 }
 211
 212 extern struct jump_entry __start___jump_table[];
 213 extern struct jump_entry __stop___jump_table[];
 214
 215 extern void jump_label_init(void);
 216 extern void jump_label_lock(void);
 217 extern void jump_label_unlock(void);
 218 extern void arch_jump_label_transform(struct jump_entry *entry,
 219                                       enum jump_label_type type);
 220 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 221                                              enum jump_label_type type);
 222 extern int jump_label_text_reserved(void *start, void *end);
 223 extern void static_key_slow_inc(struct static_key *key);
 224 extern void static_key_slow_dec(struct static_key *key);
 225 extern void static_key_slow_inc_cpuslocked(struct static_key *key);
 226 extern void static_key_slow_dec_cpuslocked(struct static_key *key);
 227 extern void jump_label_apply_nops(struct module *mod);
 228 extern int static_key_count(struct static_key *key);
 229 extern void static_key_enable(struct static_key *key);
 230 extern void static_key_disable(struct static_key *key);
 231 extern void static_key_enable_cpuslocked(struct static_key *key);
 232 extern void static_key_disable_cpuslocked(struct static_key *key);
 233
 234 /*
 235  * We should be using ATOMIC_INIT() for initializing .enabled, but
 236  * the inclusion of atomic.h is problematic for inclusion of jump_label.h
 237  * in 'low-level' headers. Thus, we are initializing .enabled with a
 238  * raw value, but have added a BUILD_BUG_ON() to catch any issues in
 239  * jump_label_init() see: kernel/jump_label.c.
 240  */
 241 #define STATIC_KEY_INIT_TRUE                                    \
 242         { .enabled = { 1 },                                     \
 243           { .entries = (void *)JUMP_TYPE_TRUE } }
 244 #define STATIC_KEY_INIT_FALSE                                   \
 245         { .enabled = { 0 },                                     \
 246           { .entries = (void *)JUMP_TYPE_FALSE } }
 247
 248 #else  /* !HAVE_JUMP_LABEL */
 249
 250 #include <linux/atomic.h>
 251 #include <linux/bug.h>
 252
 253 static inline int static_key_count(struct static_key *key)
 254 {
 255         return atomic_read(&key->enabled);
 256 }
 257
 258 static __always_inline void jump_label_init(void)
 259 {
 260         static_key_initialized = true;
 261 }
 262
 263 static __always_inline bool static_key_false(struct static_key *key)
 264 {
 265         if (unlikely(static_key_count(key) > 0))
 266                 return true;
 267         return false;
 268 }
 269
 270 static __always_inline bool static_key_true(struct static_key *key)
 271 {
 272         if (likely(static_key_count(key) > 0))
 273                 return true;
 274         return false;
 275 }
 276
 277 static inline void static_key_slow_inc(struct static_key *key)
 278 {
 279         STATIC_KEY_CHECK_USE(key);
 280         atomic_inc(&key->enabled);
 281 }
 282
 283 static inline void static_key_slow_dec(struct static_key *key)
 284 {
 285         STATIC_KEY_CHECK_USE(key);
 286         atomic_dec(&key->enabled);
 287 }
 288
 289 #define static_key_slow_inc_cpuslocked(key) static_key_slow_inc(key)
 290 #define static_key_slow_dec_cpuslocked(key) static_key_slow_dec(key)
 291
 292 static inline int jump_label_text_reserved(void *start, void *end)
 293 {
 294         return 0;
 295 }
 296
 297 static inline void jump_label_lock(void) {}
 298 static inline void jump_label_unlock(void) {}
 299
 300 static inline int jump_label_apply_nops(struct module *mod)
 301 {
 302         return 0;
 303 }
 304
 305 static inline void static_key_enable(struct static_key *key)
 306 {
 307         STATIC_KEY_CHECK_USE(key);
 308
 309         if (atomic_read(&key->enabled) != 0) {
 310                 WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
 311                 return;
 312         }
 313         atomic_set(&key->enabled, 1);
 314 }
 315
 316 static inline void static_key_disable(struct static_key *key)
 317 {
 318         STATIC_KEY_CHECK_USE(key);
 319
 320         if (atomic_read(&key->enabled) != 1) {
 321                 WARN_ON_ONCE(atomic_read(&key->enabled) != 0);
 322                 return;
 323         }
 324         atomic_set(&key->enabled, 0);
 325 }
 326
 327 #define static_key_enable_cpuslocked(k)         static_key_enable((k))
 328 #define static_key_disable_cpuslocked(k)        static_key_disable((k))
 329
 330 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
 331 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
 332
 333 #endif  /* HAVE_JUMP_LABEL */
 334
 335 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 336 #define jump_label_enabled static_key_enabled
 337
 338 /* -------------------------------------------------------------------------- */
 339
 340 /*
 341  * Two type wrappers around static_key, such that we can use compile time
 342  * type differentiation to emit the right code.
 343  *
 344  * All the below code is macros in order to play type games.
 345  */
 346
 347 struct static_key_true {
 348         struct static_key key;
 349 };
 350
 351 struct static_key_false {
 352         struct static_key key;
 353 };
 354
 355 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
 356 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
 357
 358 #define DEFINE_STATIC_KEY_TRUE(name)    \
 359         struct static_key_true name = STATIC_KEY_TRUE_INIT
 360
 361 #define DEFINE_STATIC_KEY_TRUE_RO(name) \
 362         struct static_key_true name __ro_after_init = STATIC_KEY_TRUE_INIT
 363
 364 #define DECLARE_STATIC_KEY_TRUE(name)   \
 365         extern struct static_key_true name
 366
 367 #define DEFINE_STATIC_KEY_FALSE(name)   \
 368         struct static_key_false name = STATIC_KEY_FALSE_INIT
 369
 370 #define DEFINE_STATIC_KEY_FALSE_RO(name)        \
 371         struct static_key_false name __ro_after_init = STATIC_KEY_FALSE_INIT
 372
 373 #define DECLARE_STATIC_KEY_FALSE(name)  \
 374         extern struct static_key_false name
 375
 376 #define DEFINE_STATIC_KEY_ARRAY_TRUE(name, count)               \
 377         struct static_key_true name[count] = {                  \
 378                 [0 ... (count) - 1] = STATIC_KEY_TRUE_INIT,     \
 379         }
 380
 381 #define DEFINE_STATIC_KEY_ARRAY_FALSE(name, count)              \
 382         struct static_key_false name[count] = {                 \
 383                 [0 ... (count) - 1] = STATIC_KEY_FALSE_INIT,    \
 384         }
 385
 386 extern bool ____wrong_branch_error(void);
 387
 388 #define static_key_enabled(x)                                                   \
 389 ({                                                                              \
 390         if (!__builtin_types_compatible_p(typeof(*x), struct static_key) &&     \
 391             !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
 392             !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 393                 ____wrong_branch_error();                                       \
 394         static_key_count((struct static_key *)x) > 0;                           \
 395 })
 396
 397 #ifdef HAVE_JUMP_LABEL
 398
 399 /*
 400  * Combine the right initial value (type) with the right branch order
 401  * to generate the desired result.
 402  *
 403  *
 404  * type\branch| likely (1)            | unlikely (0)
 405  * -----------+-----------------------+------------------
 406  *            |                       |
 407  *  true (1)  |    ...                |    ...
 408  *            |    NOP                |    JMP L
 409  *            |    <br-stmts>         | 1: ...
 410  *            | L: ...                |
 411  *            |                       |
 412  *            |                       | L: <br-stmts>
 413  *            |                       |    jmp 1b
 414  *            |                       |
 415  * -----------+-----------------------+------------------
 416  *            |                       |
 417  *  false (0) |    ...                |    ...
 418  *            |    JMP L              |    NOP
 419  *            |    <br-stmts>         | 1: ...
 420  *            | L: ...                |
 421  *            |                       |
 422  *            |                       | L: <br-stmts>
 423  *            |                       |    jmp 1b
 424  *            |                       |
 425  * -----------+-----------------------+------------------
 426  *
 427  * The initial value is encoded in the LSB of static_key::entries,
 428  * type: 0 = false, 1 = true.
 429  *
 430  * The branch type is encoded in the LSB of jump_entry::key,
 431  * branch: 0 = unlikely, 1 = likely.
 432  *
 433  * This gives the following logic table:
 434  *
 435  *      enabled type    branch    instuction
 436  * -----------------------------+-----------
 437  *      0       0       0       | NOP
 438  *      0       0       1       | JMP
 439  *      0       1       0       | NOP
 440  *      0       1       1       | JMP
 441  *
 442  *      1       0       0       | JMP
 443  *      1       0       1       | NOP
 444  *      1       1       0       | JMP
 445  *      1       1       1       | NOP
 446  *
 447  * Which gives the following functions:
 448  *
 449  *   dynamic: instruction = enabled ^ branch
 450  *   static:  instruction = type ^ branch
 451  *
 452  * See jump_label_type() / jump_label_init_type().
 453  */
 454
 455 #define static_branch_likely(x)                                                 \
 456 ({                                                                              \
 457         bool branch;                                                            \
 458         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 459                 branch = !arch_static_branch(&(x)->key, true);                  \
 460         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 461                 branch = !arch_static_branch_jump(&(x)->key, true);             \
 462         else                                                                    \
 463                 branch = ____wrong_branch_error();                              \
 464         likely(branch);                                                         \
 465 })
 466
 467 #define static_branch_unlikely(x)                                               \
 468 ({                                                                              \
 469         bool branch;                                                            \
 470         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 471                 branch = arch_static_branch_jump(&(x)->key, false);             \
 472         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 473                 branch = arch_static_branch(&(x)->key, false);                  \
 474         else                                                                    \
 475                 branch = ____wrong_branch_error();                              \
 476         unlikely(branch);                                                       \
 477 })
 478
 479 #else /* !HAVE_JUMP_LABEL */
 480
 481 #define static_branch_likely(x)         likely(static_key_enabled(&(x)->key))
 482 #define static_branch_unlikely(x)       unlikely(static_key_enabled(&(x)->key))
 483
 484 #endif /* HAVE_JUMP_LABEL */
 485
 486 /*
 487  * Advanced usage; refcount, branch is enabled when: count != 0
 488  */
 489
 490 #define static_branch_inc(x)            static_key_slow_inc(&(x)->key)
 491 #define static_branch_dec(x)            static_key_slow_dec(&(x)->key)
 492 #define static_branch_inc_cpuslocked(x) static_key_slow_inc_cpuslocked(&(x)->key)
 493 #define static_branch_dec_cpuslocked(x) static_key_slow_dec_cpuslocked(&(x)->key)
 494
 495 /*
 496  * Normal usage; boolean enable/disable.
 497  */
 498
 499 #define static_branch_enable(x)                 static_key_enable(&(x)->key)
 500 #define static_branch_disable(x)                static_key_disable(&(x)->key)
 501 #define static_branch_enable_cpuslocked(x)      static_key_enable_cpuslocked(&(x)->key)
 502 #define static_branch_disable_cpuslocked(x)     static_key_disable_cpuslocked(&(x)->key)
 503
 504 #endif /* __ASSEMBLY__ */
 505
 506 #endif  /* _LINUX_JUMP_LABEL_H */