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 <[email protected]> 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 /* 93 * Note: 94 * To make anonymous unions work with old compilers, the static 95 * initialization of them requires brackets. This creates a dependency 96 * on the order of the struct with the initializers. If any fields 97 * are added, STATIC_KEY_INIT_TRUE and STATIC_KEY_INIT_FALSE may need 98 * to be modified. 99 * 100 * bit 0 => 1 if key is initially true 101 * 0 if initially false 102 * bit 1 => 1 if points to struct static_key_mod 103 * 0 if points to struct jump_entry 104 */ 105 union { 106 unsigned long type; 107 struct jump_entry *entries; 108 struct static_key_mod *next; 109 }; 110 }; 111 112 #else 113 struct static_key { 114 atomic_t enabled; 115 }; 116 #endif /* HAVE_JUMP_LABEL */ 117 #endif /* __ASSEMBLY__ */ 118 119 #ifdef HAVE_JUMP_LABEL 120 #include <asm/jump_label.h> 121 #endif 122 123 #ifndef __ASSEMBLY__ 124 125 enum jump_label_type { 126 JUMP_LABEL_NOP = 0, 127 JUMP_LABEL_JMP, 128 }; 129 130 struct module; 131 132 #ifdef HAVE_JUMP_LABEL 133 134 #define JUMP_TYPE_FALSE 0UL 135 #define JUMP_TYPE_TRUE 1UL 136 #define JUMP_TYPE_LINKED 2UL 137 #define JUMP_TYPE_MASK 3UL 138 139 static __always_inline bool static_key_false(struct static_key *key) 140 { 141 return arch_static_branch(key, false); 142 } 143 144 static __always_inline bool static_key_true(struct static_key *key) 145 { 146 return !arch_static_branch(key, true); 147 } 148 149 extern struct jump_entry __start___jump_table[]; 150 extern struct jump_entry __stop___jump_table[]; 151 152 extern void jump_label_init(void); 153 extern void jump_label_lock(void); 154 extern void jump_label_unlock(void); 155 extern void arch_jump_label_transform(struct jump_entry *entry, 156 enum jump_label_type type); 157 extern void arch_jump_label_transform_static(struct jump_entry *entry, 158 enum jump_label_type type); 159 extern int jump_label_text_reserved(void *start, void *end); 160 extern void static_key_slow_inc(struct static_key *key); 161 extern void static_key_slow_dec(struct static_key *key); 162 extern void jump_label_apply_nops(struct module *mod); 163 extern int static_key_count(struct static_key *key); 164 extern void static_key_enable(struct static_key *key); 165 extern void static_key_disable(struct static_key *key); 166 167 /* 168 * We should be using ATOMIC_INIT() for initializing .enabled, but 169 * the inclusion of atomic.h is problematic for inclusion of jump_label.h 170 * in 'low-level' headers. Thus, we are initializing .enabled with a 171 * raw value, but have added a BUILD_BUG_ON() to catch any issues in 172 * jump_label_init() see: kernel/jump_label.c. 173 */ 174 #define STATIC_KEY_INIT_TRUE \ 175 { .enabled = { 1 }, \ 176 { .entries = (void *)JUMP_TYPE_TRUE } } 177 #define STATIC_KEY_INIT_FALSE \ 178 { .enabled = { 0 }, \ 179 { .entries = (void *)JUMP_TYPE_FALSE } } 180 181 #else /* !HAVE_JUMP_LABEL */ 182 183 #include <linux/atomic.h> 184 #include <linux/bug.h> 185 186 static inline int static_key_count(struct static_key *key) 187 { 188 return atomic_read(&key->enabled); 189 } 190 191 static __always_inline void jump_label_init(void) 192 { 193 static_key_initialized = true; 194 } 195 196 static __always_inline bool static_key_false(struct static_key *key) 197 { 198 if (unlikely(static_key_count(key) > 0)) 199 return true; 200 return false; 201 } 202 203 static __always_inline bool static_key_true(struct static_key *key) 204 { 205 if (likely(static_key_count(key) > 0)) 206 return true; 207 return false; 208 } 209 210 static inline void static_key_slow_inc(struct static_key *key) 211 { 212 STATIC_KEY_CHECK_USE(); 213 atomic_inc(&key->enabled); 214 } 215 216 static inline void static_key_slow_dec(struct static_key *key) 217 { 218 STATIC_KEY_CHECK_USE(); 219 atomic_dec(&key->enabled); 220 } 221 222 static inline int jump_label_text_reserved(void *start, void *end) 223 { 224 return 0; 225 } 226 227 static inline void jump_label_lock(void) {} 228 static inline void jump_label_unlock(void) {} 229 230 static inline int jump_label_apply_nops(struct module *mod) 231 { 232 return 0; 233 } 234 235 static inline void static_key_enable(struct static_key *key) 236 { 237 int count = static_key_count(key); 238 239 WARN_ON_ONCE(count < 0 || count > 1); 240 241 if (!count) 242 static_key_slow_inc(key); 243 } 244 245 static inline void static_key_disable(struct static_key *key) 246 { 247 int count = static_key_count(key); 248 249 WARN_ON_ONCE(count < 0 || count > 1); 250 251 if (count) 252 static_key_slow_dec(key); 253 } 254 255 #define STATIC_KEY_INIT_TRUE { .enabled = ATOMIC_INIT(1) } 256 #define STATIC_KEY_INIT_FALSE { .enabled = ATOMIC_INIT(0) } 257 258 #endif /* HAVE_JUMP_LABEL */ 259 260 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE 261 #define jump_label_enabled static_key_enabled 262 263 /* -------------------------------------------------------------------------- */ 264 265 /* 266 * Two type wrappers around static_key, such that we can use compile time 267 * type differentiation to emit the right code. 268 * 269 * All the below code is macros in order to play type games. 270 */ 271 272 struct static_key_true { 273 struct static_key key; 274 }; 275 276 struct static_key_false { 277 struct static_key key; 278 }; 279 280 #define STATIC_KEY_TRUE_INIT (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE, } 281 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, } 282 283 #define DEFINE_STATIC_KEY_TRUE(name) \ 284 struct static_key_true name = STATIC_KEY_TRUE_INIT 285 286 #define DECLARE_STATIC_KEY_TRUE(name) \ 287 extern struct static_key_true name 288 289 #define DEFINE_STATIC_KEY_FALSE(name) \ 290 struct static_key_false name = STATIC_KEY_FALSE_INIT 291 292 #define DECLARE_STATIC_KEY_FALSE(name) \ 293 extern struct static_key_false name 294 295 #define DEFINE_STATIC_KEY_ARRAY_TRUE(name, count) \ 296 struct static_key_true name[count] = { \ 297 [0 ... (count) - 1] = STATIC_KEY_TRUE_INIT, \ 298 } 299 300 #define DEFINE_STATIC_KEY_ARRAY_FALSE(name, count) \ 301 struct static_key_false name[count] = { \ 302 [0 ... (count) - 1] = STATIC_KEY_FALSE_INIT, \ 303 } 304 305 extern bool ____wrong_branch_error(void); 306 307 #define static_key_enabled(x) \ 308 ({ \ 309 if (!__builtin_types_compatible_p(typeof(*x), struct static_key) && \ 310 !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\ 311 !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \ 312 ____wrong_branch_error(); \ 313 static_key_count((struct static_key *)x) > 0; \ 314 }) 315 316 #ifdef HAVE_JUMP_LABEL 317 318 /* 319 * Combine the right initial value (type) with the right branch order 320 * to generate the desired result. 321 * 322 * 323 * type\branch| likely (1) | unlikely (0) 324 * -----------+-----------------------+------------------ 325 * | | 326 * true (1) | ... | ... 327 * | NOP | JMP L 328 * | <br-stmts> | 1: ... 329 * | L: ... | 330 * | | 331 * | | L: <br-stmts> 332 * | | jmp 1b 333 * | | 334 * -----------+-----------------------+------------------ 335 * | | 336 * false (0) | ... | ... 337 * | JMP L | NOP 338 * | <br-stmts> | 1: ... 339 * | L: ... | 340 * | | 341 * | | L: <br-stmts> 342 * | | jmp 1b 343 * | | 344 * -----------+-----------------------+------------------ 345 * 346 * The initial value is encoded in the LSB of static_key::entries, 347 * type: 0 = false, 1 = true. 348 * 349 * The branch type is encoded in the LSB of jump_entry::key, 350 * branch: 0 = unlikely, 1 = likely. 351 * 352 * This gives the following logic table: 353 * 354 * enabled type branch instuction 355 * -----------------------------+----------- 356 * 0 0 0 | NOP 357 * 0 0 1 | JMP 358 * 0 1 0 | NOP 359 * 0 1 1 | JMP 360 * 361 * 1 0 0 | JMP 362 * 1 0 1 | NOP 363 * 1 1 0 | JMP 364 * 1 1 1 | NOP 365 * 366 * Which gives the following functions: 367 * 368 * dynamic: instruction = enabled ^ branch 369 * static: instruction = type ^ branch 370 * 371 * See jump_label_type() / jump_label_init_type(). 372 */ 373 374 #define static_branch_likely(x) \ 375 ({ \ 376 bool branch; \ 377 if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \ 378 branch = !arch_static_branch(&(x)->key, true); \ 379 else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \ 380 branch = !arch_static_branch_jump(&(x)->key, true); \ 381 else \ 382 branch = ____wrong_branch_error(); \ 383 branch; \ 384 }) 385 386 #define static_branch_unlikely(x) \ 387 ({ \ 388 bool branch; \ 389 if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \ 390 branch = arch_static_branch_jump(&(x)->key, false); \ 391 else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \ 392 branch = arch_static_branch(&(x)->key, false); \ 393 else \ 394 branch = ____wrong_branch_error(); \ 395 branch; \ 396 }) 397 398 #else /* !HAVE_JUMP_LABEL */ 399 400 #define static_branch_likely(x) likely(static_key_enabled(&(x)->key)) 401 #define static_branch_unlikely(x) unlikely(static_key_enabled(&(x)->key)) 402 403 #endif /* HAVE_JUMP_LABEL */ 404 405 /* 406 * Advanced usage; refcount, branch is enabled when: count != 0 407 */ 408 409 #define static_branch_inc(x) static_key_slow_inc(&(x)->key) 410 #define static_branch_dec(x) static_key_slow_dec(&(x)->key) 411 412 /* 413 * Normal usage; boolean enable/disable. 414 */ 415 416 #define static_branch_enable(x) static_key_enable(&(x)->key) 417 #define static_branch_disable(x) static_key_disable(&(x)->key) 418 419 #endif /* __ASSEMBLY__ */ 420 421 #endif /* _LINUX_JUMP_LABEL_H */ 422