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 Peter Zijlstra <[email protected]> 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 bool static_key_enabled(struct static_key *key) 220 { 221 return static_key_count(key) > 0; 222 } 223 224 static inline void static_key_enable(struct static_key *key) 225 { 226 int count = static_key_count(key); 227 228 WARN_ON_ONCE(count < 0 || count > 1); 229 230 if (!count) 231 static_key_slow_inc(key); 232 } 233 234 static inline void static_key_disable(struct static_key *key) 235 { 236 int count = static_key_count(key); 237 238 WARN_ON_ONCE(count < 0 || count > 1); 239 240 if (count) 241 static_key_slow_dec(key); 242 } 243 244 /* -------------------------------------------------------------------------- */ 245 246 /* 247 * Two type wrappers around static_key, such that we can use compile time 248 * type differentiation to emit the right code. 249 * 250 * All the below code is macros in order to play type games. 251 */ 252 253 struct static_key_true { 254 struct static_key key; 255 }; 256 257 struct static_key_false { 258 struct static_key key; 259 }; 260 261 #define STATIC_KEY_TRUE_INIT (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE, } 262 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, } 263 264 #define DEFINE_STATIC_KEY_TRUE(name) \ 265 struct static_key_true name = STATIC_KEY_TRUE_INIT 266 267 #define DEFINE_STATIC_KEY_FALSE(name) \ 268 struct static_key_false name = STATIC_KEY_FALSE_INIT 269 270 #ifdef HAVE_JUMP_LABEL 271 272 /* 273 * Combine the right initial value (type) with the right branch order 274 * to generate the desired result. 275 * 276 * 277 * type\branch| likely (1) | unlikely (0) 278 * -----------+-----------------------+------------------ 279 * | | 280 * true (1) | ... | ... 281 * | NOP | JMP L 282 * | <br-stmts> | 1: ... 283 * | L: ... | 284 * | | 285 * | | L: <br-stmts> 286 * | | jmp 1b 287 * | | 288 * -----------+-----------------------+------------------ 289 * | | 290 * false (0) | ... | ... 291 * | JMP L | NOP 292 * | <br-stmts> | 1: ... 293 * | L: ... | 294 * | | 295 * | | L: <br-stmts> 296 * | | jmp 1b 297 * | | 298 * -----------+-----------------------+------------------ 299 * 300 * The initial value is encoded in the LSB of static_key::entries, 301 * type: 0 = false, 1 = true. 302 * 303 * The branch type is encoded in the LSB of jump_entry::key, 304 * branch: 0 = unlikely, 1 = likely. 305 * 306 * This gives the following logic table: 307 * 308 * enabled type branch instuction 309 * -----------------------------+----------- 310 * 0 0 0 | NOP 311 * 0 0 1 | JMP 312 * 0 1 0 | NOP 313 * 0 1 1 | JMP 314 * 315 * 1 0 0 | JMP 316 * 1 0 1 | NOP 317 * 1 1 0 | JMP 318 * 1 1 1 | NOP 319 * 320 * Which gives the following functions: 321 * 322 * dynamic: instruction = enabled ^ branch 323 * static: instruction = type ^ branch 324 * 325 * See jump_label_type() / jump_label_init_type(). 326 */ 327 328 extern bool ____wrong_branch_error(void); 329 330 #define static_branch_likely(x) \ 331 ({ \ 332 bool branch; \ 333 if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \ 334 branch = !arch_static_branch(&(x)->key, true); \ 335 else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \ 336 branch = !arch_static_branch_jump(&(x)->key, true); \ 337 else \ 338 branch = ____wrong_branch_error(); \ 339 branch; \ 340 }) 341 342 #define static_branch_unlikely(x) \ 343 ({ \ 344 bool branch; \ 345 if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \ 346 branch = arch_static_branch_jump(&(x)->key, false); \ 347 else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \ 348 branch = arch_static_branch(&(x)->key, false); \ 349 else \ 350 branch = ____wrong_branch_error(); \ 351 branch; \ 352 }) 353 354 #else /* !HAVE_JUMP_LABEL */ 355 356 #define static_branch_likely(x) likely(static_key_enabled(&(x)->key)) 357 #define static_branch_unlikely(x) unlikely(static_key_enabled(&(x)->key)) 358 359 #endif /* HAVE_JUMP_LABEL */ 360 361 /* 362 * Advanced usage; refcount, branch is enabled when: count != 0 363 */ 364 365 #define static_branch_inc(x) static_key_slow_inc(&(x)->key) 366 #define static_branch_dec(x) static_key_slow_dec(&(x)->key) 367 368 /* 369 * Normal usage; boolean enable/disable. 370 */ 371 372 #define static_branch_enable(x) static_key_enable(&(x)->key) 373 #define static_branch_disable(x) static_key_disable(&(x)->key) 374 375 #endif /* _LINUX_JUMP_LABEL_H */ 376 377 #endif /* __ASSEMBLY__ */ 378