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