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