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 /* 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__ */ 408