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