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 static_key_slow_inc_cpuslocked(struct static_key *key);
 164 extern void static_key_slow_dec_cpuslocked(struct static_key *key);
 165 extern void jump_label_apply_nops(struct module *mod);
 166 extern int static_key_count(struct static_key *key);
 167 extern void static_key_enable(struct static_key *key);
 168 extern void static_key_disable(struct static_key *key);
 169 extern void static_key_enable_cpuslocked(struct static_key *key);
 170 extern void static_key_disable_cpuslocked(struct static_key *key);
 171
 172 /*
 173  * We should be using ATOMIC_INIT() for initializing .enabled, but
 174  * the inclusion of atomic.h is problematic for inclusion of jump_label.h
 175  * in 'low-level' headers. Thus, we are initializing .enabled with a
 176  * raw value, but have added a BUILD_BUG_ON() to catch any issues in
 177  * jump_label_init() see: kernel/jump_label.c.
 178  */
 179 #define STATIC_KEY_INIT_TRUE                                    \
 180         { .enabled = { 1 },                                     \
 181           { .entries = (void *)JUMP_TYPE_TRUE } }
 182 #define STATIC_KEY_INIT_FALSE                                   \
 183         { .enabled = { 0 },                                     \
 184           { .entries = (void *)JUMP_TYPE_FALSE } }
 185
 186 #else  /* !HAVE_JUMP_LABEL */
 187
 188 #include <linux/atomic.h>
 189 #include <linux/bug.h>
 190
 191 static inline int static_key_count(struct static_key *key)
 192 {
 193         return atomic_read(&key->enabled);
 194 }
 195
 196 static __always_inline void jump_label_init(void)
 197 {
 198         static_key_initialized = true;
 199 }
 200
 201 static __always_inline bool static_key_false(struct static_key *key)
 202 {
 203         if (unlikely(static_key_count(key) > 0))
 204                 return true;
 205         return false;
 206 }
 207
 208 static __always_inline bool static_key_true(struct static_key *key)
 209 {
 210         if (likely(static_key_count(key) > 0))
 211                 return true;
 212         return false;
 213 }
 214
 215 static inline void static_key_slow_inc(struct static_key *key)
 216 {
 217         STATIC_KEY_CHECK_USE(key);
 218         atomic_inc(&key->enabled);
 219 }
 220
 221 static inline void static_key_slow_dec(struct static_key *key)
 222 {
 223         STATIC_KEY_CHECK_USE(key);
 224         atomic_dec(&key->enabled);
 225 }
 226
 227 #define static_key_slow_inc_cpuslocked(key) static_key_slow_inc(key)
 228 #define static_key_slow_dec_cpuslocked(key) static_key_slow_dec(key)
 229
 230 static inline int jump_label_text_reserved(void *start, void *end)
 231 {
 232         return 0;
 233 }
 234
 235 static inline void jump_label_lock(void) {}
 236 static inline void jump_label_unlock(void) {}
 237
 238 static inline int jump_label_apply_nops(struct module *mod)
 239 {
 240         return 0;
 241 }
 242
 243 static inline void static_key_enable(struct static_key *key)
 244 {
 245         STATIC_KEY_CHECK_USE(key);
 246
 247         if (atomic_read(&key->enabled) != 0) {
 248                 WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
 249                 return;
 250         }
 251         atomic_set(&key->enabled, 1);
 252 }
 253
 254 static inline void static_key_disable(struct static_key *key)
 255 {
 256         STATIC_KEY_CHECK_USE(key);
 257
 258         if (atomic_read(&key->enabled) != 1) {
 259                 WARN_ON_ONCE(atomic_read(&key->enabled) != 0);
 260                 return;
 261         }
 262         atomic_set(&key->enabled, 0);
 263 }
 264
 265 #define static_key_enable_cpuslocked(k)         static_key_enable((k))
 266 #define static_key_disable_cpuslocked(k)        static_key_disable((k))
 267
 268 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
 269 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
 270
 271 #endif  /* HAVE_JUMP_LABEL */
 272
 273 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 274 #define jump_label_enabled static_key_enabled
 275
 276 /* -------------------------------------------------------------------------- */
 277
 278 /*
 279  * Two type wrappers around static_key, such that we can use compile time
 280  * type differentiation to emit the right code.
 281  *
 282  * All the below code is macros in order to play type games.
 283  */
 284
 285 struct static_key_true {
 286         struct static_key key;
 287 };
 288
 289 struct static_key_false {
 290         struct static_key key;
 291 };
 292
 293 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
 294 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
 295
 296 #define DEFINE_STATIC_KEY_TRUE(name)    \
 297         struct static_key_true name = STATIC_KEY_TRUE_INIT
 298
 299 #define DECLARE_STATIC_KEY_TRUE(name)   \
 300         extern struct static_key_true name
 301
 302 #define DEFINE_STATIC_KEY_FALSE(name)   \
 303         struct static_key_false name = STATIC_KEY_FALSE_INIT
 304
 305 #define DECLARE_STATIC_KEY_FALSE(name)  \
 306         extern struct static_key_false name
 307
 308 #define DEFINE_STATIC_KEY_ARRAY_TRUE(name, count)               \
 309         struct static_key_true name[count] = {                  \
 310                 [0 ... (count) - 1] = STATIC_KEY_TRUE_INIT,     \
 311         }
 312
 313 #define DEFINE_STATIC_KEY_ARRAY_FALSE(name, count)              \
 314         struct static_key_false name[count] = {                 \
 315                 [0 ... (count) - 1] = STATIC_KEY_FALSE_INIT,    \
 316         }
 317
 318 extern bool ____wrong_branch_error(void);
 319
 320 #define static_key_enabled(x)                                                   \
 321 ({                                                                              \
 322         if (!__builtin_types_compatible_p(typeof(*x), struct static_key) &&     \
 323             !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
 324             !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 325                 ____wrong_branch_error();                                       \
 326         static_key_count((struct static_key *)x) > 0;                           \
 327 })
 328
 329 #ifdef HAVE_JUMP_LABEL
 330
 331 /*
 332  * Combine the right initial value (type) with the right branch order
 333  * to generate the desired result.
 334  *
 335  *
 336  * type\branch| likely (1)            | unlikely (0)
 337  * -----------+-----------------------+------------------
 338  *            |                       |
 339  *  true (1)  |    ...                |    ...
 340  *            |    NOP                |    JMP L
 341  *            |    <br-stmts>         | 1: ...
 342  *            | L: ...                |
 343  *            |                       |
 344  *            |                       | L: <br-stmts>
 345  *            |                       |    jmp 1b
 346  *            |                       |
 347  * -----------+-----------------------+------------------
 348  *            |                       |
 349  *  false (0) |    ...                |    ...
 350  *            |    JMP L              |    NOP
 351  *            |    <br-stmts>         | 1: ...
 352  *            | L: ...                |
 353  *            |                       |
 354  *            |                       | L: <br-stmts>
 355  *            |                       |    jmp 1b
 356  *            |                       |
 357  * -----------+-----------------------+------------------
 358  *
 359  * The initial value is encoded in the LSB of static_key::entries,
 360  * type: 0 = false, 1 = true.
 361  *
 362  * The branch type is encoded in the LSB of jump_entry::key,
 363  * branch: 0 = unlikely, 1 = likely.
 364  *
 365  * This gives the following logic table:
 366  *
 367  *      enabled type    branch    instuction
 368  * -----------------------------+-----------
 369  *      0       0       0       | NOP
 370  *      0       0       1       | JMP
 371  *      0       1       0       | NOP
 372  *      0       1       1       | JMP
 373  *
 374  *      1       0       0       | JMP
 375  *      1       0       1       | NOP
 376  *      1       1       0       | JMP
 377  *      1       1       1       | NOP
 378  *
 379  * Which gives the following functions:
 380  *
 381  *   dynamic: instruction = enabled ^ branch
 382  *   static:  instruction = type ^ branch
 383  *
 384  * See jump_label_type() / jump_label_init_type().
 385  */
 386
 387 #define static_branch_likely(x)                                                 \
 388 ({                                                                              \
 389         bool branch;                                                            \
 390         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 391                 branch = !arch_static_branch(&(x)->key, true);                  \
 392         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 393                 branch = !arch_static_branch_jump(&(x)->key, true);             \
 394         else                                                                    \
 395                 branch = ____wrong_branch_error();                              \
 396         likely(branch);                                                         \
 397 })
 398
 399 #define static_branch_unlikely(x)                                               \
 400 ({                                                                              \
 401         bool branch;                                                            \
 402         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 403                 branch = arch_static_branch_jump(&(x)->key, false);             \
 404         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 405                 branch = arch_static_branch(&(x)->key, false);                  \
 406         else                                                                    \
 407                 branch = ____wrong_branch_error();                              \
 408         unlikely(branch);                                                       \
 409 })
 410
 411 #else /* !HAVE_JUMP_LABEL */
 412
 413 #define static_branch_likely(x)         likely(static_key_enabled(&(x)->key))
 414 #define static_branch_unlikely(x)       unlikely(static_key_enabled(&(x)->key))
 415
 416 #endif /* HAVE_JUMP_LABEL */
 417
 418 /*
 419  * Advanced usage; refcount, branch is enabled when: count != 0
 420  */
 421
 422 #define static_branch_inc(x)            static_key_slow_inc(&(x)->key)
 423 #define static_branch_dec(x)            static_key_slow_dec(&(x)->key)
 424 #define static_branch_inc_cpuslocked(x) static_key_slow_inc_cpuslocked(&(x)->key)
 425 #define static_branch_dec_cpuslocked(x) static_key_slow_dec_cpuslocked(&(x)->key)
 426
 427 /*
 428  * Normal usage; boolean enable/disable.
 429  */
 430
 431 #define static_branch_enable(x)                 static_key_enable(&(x)->key)
 432 #define static_branch_disable(x)                static_key_disable(&(x)->key)
 433 #define static_branch_enable_cpuslocked(x)      static_key_enable_cpuslocked(&(x)->key)
 434 #define static_branch_disable_cpuslocked(x)     static_key_disable_cpuslocked(&(x)->key)
 435
 436 #endif /* __ASSEMBLY__ */
 437
 438 #endif  /* _LINUX_JUMP_LABEL_H */