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
  80 extern bool static_key_initialized;
  81
  82 #define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized,                  \
  83                                     "%s used before call to jump_label_init", \
  84                                     __func__)
  85
  86 #ifdef HAVE_JUMP_LABEL
  87
  88 struct static_key {
  89         atomic_t enabled;
  90 /* Set lsb bit to 1 if branch is default true, 0 ot */
  91         struct jump_entry *entries;
  92 #ifdef CONFIG_MODULES
  93         struct static_key_mod *next;
  94 #endif
  95 };
  96
  97 #else
  98 struct static_key {
  99         atomic_t enabled;
 100 };
 101 #endif  /* HAVE_JUMP_LABEL */
 102 #endif /* __ASSEMBLY__ */
 103
 104 #ifdef HAVE_JUMP_LABEL
 105 #include <asm/jump_label.h>
 106 #endif
 107
 108 #ifndef __ASSEMBLY__
 109
 110 enum jump_label_type {
 111         JUMP_LABEL_NOP = 0,
 112         JUMP_LABEL_JMP,
 113 };
 114
 115 struct module;
 116
 117 #ifdef HAVE_JUMP_LABEL
 118
 119 #define JUMP_TYPE_FALSE 0UL
 120 #define JUMP_TYPE_TRUE  1UL
 121 #define JUMP_TYPE_MASK  1UL
 122
 123 static __always_inline bool static_key_false(struct static_key *key)
 124 {
 125         return arch_static_branch(key, false);
 126 }
 127
 128 static __always_inline bool static_key_true(struct static_key *key)
 129 {
 130         return !arch_static_branch(key, true);
 131 }
 132
 133 extern struct jump_entry __start___jump_table[];
 134 extern struct jump_entry __stop___jump_table[];
 135
 136 extern void jump_label_init(void);
 137 extern void jump_label_lock(void);
 138 extern void jump_label_unlock(void);
 139 extern void arch_jump_label_transform(struct jump_entry *entry,
 140                                       enum jump_label_type type);
 141 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 142                                              enum jump_label_type type);
 143 extern int jump_label_text_reserved(void *start, void *end);
 144 extern void static_key_slow_inc(struct static_key *key);
 145 extern void static_key_slow_dec(struct static_key *key);
 146 extern void jump_label_apply_nops(struct module *mod);
 147 extern int static_key_count(struct static_key *key);
 148 extern void static_key_enable(struct static_key *key);
 149 extern void static_key_disable(struct static_key *key);
 150
 151 /*
 152  * We should be using ATOMIC_INIT() for initializing .enabled, but
 153  * the inclusion of atomic.h is problematic for inclusion of jump_label.h
 154  * in 'low-level' headers. Thus, we are initializing .enabled with a
 155  * raw value, but have added a BUILD_BUG_ON() to catch any issues in
 156  * jump_label_init() see: kernel/jump_label.c.
 157  */
 158 #define STATIC_KEY_INIT_TRUE                                    \
 159         { .enabled = { 1 },                                     \
 160           .entries = (void *)JUMP_TYPE_TRUE }
 161 #define STATIC_KEY_INIT_FALSE                                   \
 162         { .enabled = { 0 },                                     \
 163           .entries = (void *)JUMP_TYPE_FALSE }
 164
 165 #else  /* !HAVE_JUMP_LABEL */
 166
 167 #include <linux/atomic.h>
 168 #include <linux/bug.h>
 169
 170 static inline int static_key_count(struct static_key *key)
 171 {
 172         return atomic_read(&key->enabled);
 173 }
 174
 175 static __always_inline void jump_label_init(void)
 176 {
 177         static_key_initialized = true;
 178 }
 179
 180 static __always_inline bool static_key_false(struct static_key *key)
 181 {
 182         if (unlikely(static_key_count(key) > 0))
 183                 return true;
 184         return false;
 185 }
 186
 187 static __always_inline bool static_key_true(struct static_key *key)
 188 {
 189         if (likely(static_key_count(key) > 0))
 190                 return true;
 191         return false;
 192 }
 193
 194 static inline void static_key_slow_inc(struct static_key *key)
 195 {
 196         STATIC_KEY_CHECK_USE();
 197         atomic_inc(&key->enabled);
 198 }
 199
 200 static inline void static_key_slow_dec(struct static_key *key)
 201 {
 202         STATIC_KEY_CHECK_USE();
 203         atomic_dec(&key->enabled);
 204 }
 205
 206 static inline int jump_label_text_reserved(void *start, void *end)
 207 {
 208         return 0;
 209 }
 210
 211 static inline void jump_label_lock(void) {}
 212 static inline void jump_label_unlock(void) {}
 213
 214 static inline int jump_label_apply_nops(struct module *mod)
 215 {
 216         return 0;
 217 }
 218
 219 static inline void static_key_enable(struct static_key *key)
 220 {
 221         int count = static_key_count(key);
 222
 223         WARN_ON_ONCE(count < 0 || count > 1);
 224
 225         if (!count)
 226                 static_key_slow_inc(key);
 227 }
 228
 229 static inline void static_key_disable(struct static_key *key)
 230 {
 231         int count = static_key_count(key);
 232
 233         WARN_ON_ONCE(count < 0 || count > 1);
 234
 235         if (count)
 236                 static_key_slow_dec(key);
 237 }
 238
 239 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
 240 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
 241
 242 #endif  /* HAVE_JUMP_LABEL */
 243
 244 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
 245 #define jump_label_enabled static_key_enabled
 246
 247 /* -------------------------------------------------------------------------- */
 248
 249 /*
 250  * Two type wrappers around static_key, such that we can use compile time
 251  * type differentiation to emit the right code.
 252  *
 253  * All the below code is macros in order to play type games.
 254  */
 255
 256 struct static_key_true {
 257         struct static_key key;
 258 };
 259
 260 struct static_key_false {
 261         struct static_key key;
 262 };
 263
 264 #define STATIC_KEY_TRUE_INIT  (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE,  }
 265 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
 266
 267 #define DEFINE_STATIC_KEY_TRUE(name)    \
 268         struct static_key_true name = STATIC_KEY_TRUE_INIT
 269
 270 #define DEFINE_STATIC_KEY_FALSE(name)   \
 271         struct static_key_false name = STATIC_KEY_FALSE_INIT
 272
 273 extern bool ____wrong_branch_error(void);
 274
 275 #define static_key_enabled(x)                                                   \
 276 ({                                                                              \
 277         if (!__builtin_types_compatible_p(typeof(*x), struct static_key) &&     \
 278             !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
 279             !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 280                 ____wrong_branch_error();                                       \
 281         static_key_count((struct static_key *)x) > 0;                           \
 282 })
 283
 284 #ifdef HAVE_JUMP_LABEL
 285
 286 /*
 287  * Combine the right initial value (type) with the right branch order
 288  * to generate the desired result.
 289  *
 290  *
 291  * type\branch| likely (1)            | unlikely (0)
 292  * -----------+-----------------------+------------------
 293  *            |                       |
 294  *  true (1)  |    ...                |    ...
 295  *            |    NOP                |    JMP L
 296  *            |    <br-stmts>         | 1: ...
 297  *            | L: ...                |
 298  *            |                       |
 299  *            |                       | L: <br-stmts>
 300  *            |                       |    jmp 1b
 301  *            |                       |
 302  * -----------+-----------------------+------------------
 303  *            |                       |
 304  *  false (0) |    ...                |    ...
 305  *            |    JMP L              |    NOP
 306  *            |    <br-stmts>         | 1: ...
 307  *            | L: ...                |
 308  *            |                       |
 309  *            |                       | L: <br-stmts>
 310  *            |                       |    jmp 1b
 311  *            |                       |
 312  * -----------+-----------------------+------------------
 313  *
 314  * The initial value is encoded in the LSB of static_key::entries,
 315  * type: 0 = false, 1 = true.
 316  *
 317  * The branch type is encoded in the LSB of jump_entry::key,
 318  * branch: 0 = unlikely, 1 = likely.
 319  *
 320  * This gives the following logic table:
 321  *
 322  *      enabled type    branch    instuction
 323  * -----------------------------+-----------
 324  *      0       0       0       | NOP
 325  *      0       0       1       | JMP
 326  *      0       1       0       | NOP
 327  *      0       1       1       | JMP
 328  *
 329  *      1       0       0       | JMP
 330  *      1       0       1       | NOP
 331  *      1       1       0       | JMP
 332  *      1       1       1       | NOP
 333  *
 334  * Which gives the following functions:
 335  *
 336  *   dynamic: instruction = enabled ^ branch
 337  *   static:  instruction = type ^ branch
 338  *
 339  * See jump_label_type() / jump_label_init_type().
 340  */
 341
 342 #define static_branch_likely(x)                                                 \
 343 ({                                                                              \
 344         bool branch;                                                            \
 345         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 346                 branch = !arch_static_branch(&(x)->key, true);                  \
 347         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 348                 branch = !arch_static_branch_jump(&(x)->key, true);             \
 349         else                                                                    \
 350                 branch = ____wrong_branch_error();                              \
 351         branch;                                                                 \
 352 })
 353
 354 #define static_branch_unlikely(x)                                               \
 355 ({                                                                              \
 356         bool branch;                                                            \
 357         if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))   \
 358                 branch = arch_static_branch_jump(&(x)->key, false);             \
 359         else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
 360                 branch = arch_static_branch(&(x)->key, false);                  \
 361         else                                                                    \
 362                 branch = ____wrong_branch_error();                              \
 363         branch;                                                                 \
 364 })
 365
 366 #else /* !HAVE_JUMP_LABEL */
 367
 368 #define static_branch_likely(x)         likely(static_key_enabled(&(x)->key))
 369 #define static_branch_unlikely(x)       unlikely(static_key_enabled(&(x)->key))
 370
 371 #endif /* HAVE_JUMP_LABEL */
 372
 373 /*
 374  * Advanced usage; refcount, branch is enabled when: count != 0
 375  */
 376
 377 #define static_branch_inc(x)            static_key_slow_inc(&(x)->key)
 378 #define static_branch_dec(x)            static_key_slow_dec(&(x)->key)
 379
 380 /*
 381  * Normal usage; boolean enable/disable.
 382  */
 383
 384 #define static_branch_enable(x)         static_key_enable(&(x)->key)
 385 #define static_branch_disable(x)        static_key_disable(&(x)->key)
 386
 387 #endif  /* _LINUX_JUMP_LABEL_H */
 388
 389 #endif /* __ASSEMBLY__ */