1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _LINUX_JUMP_LABEL_H
3 #define _LINUX_JUMP_LABEL_H
4
5 /*
6 * Jump label support
7 *
8 * Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
9 * Copyright (C) 2011-2012 Red Hat, Inc., Peter Zijlstra
10 *
11 * DEPRECATED API:
12 *
13 * The use of 'struct static_key' directly, is now DEPRECATED. In addition
14 * static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
15 *
16 * struct static_key false = STATIC_KEY_INIT_FALSE;
17 * struct static_key true = STATIC_KEY_INIT_TRUE;
18 * static_key_true()
19 * static_key_false()
20 *
21 * The updated API replacements are:
22 *
23 * DEFINE_STATIC_KEY_TRUE(key);
24 * DEFINE_STATIC_KEY_FALSE(key);
25 * DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
26 * DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count);
27 * static_branch_likely()
28 * static_branch_unlikely()
29 *
30 * Jump labels provide an interface to generate dynamic branches using
31 * self-modifying code. Assuming toolchain and architecture support, if we
32 * define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
33 * an "if (static_branch_unlikely(&key))" statement is an unconditional branch
34 * (which defaults to false - and the true block is placed out of line).
35 * Similarly, we can define an initially true key via
36 * "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
37 * "if (static_branch_unlikely(&key))", in which case we will generate an
38 * unconditional branch to the out-of-line true branch. Keys that are
39 * initially true or false can be using in both static_branch_unlikely()
40 * and static_branch_likely() statements.
41 *
42 * At runtime we can change the branch target by setting the key
43 * to true via a call to static_branch_enable(), or false using
44 * static_branch_disable(). If the direction of the branch is switched by
45 * these calls then we run-time modify the branch target via a
46 * no-op -> jump or jump -> no-op conversion. For example, for an
47 * initially false key that is used in an "if (static_branch_unlikely(&key))"
48 * statement, setting the key to true requires us to patch in a jump
49 * to the out-of-line of true branch.
50 *
51 * In addition to static_branch_{enable,disable}, we can also reference count
52 * the key or branch direction via static_branch_{inc,dec}. Thus,
53 * static_branch_inc() can be thought of as a 'make more true' and
54 * static_branch_dec() as a 'make more false'.
55 *
56 * Since this relies on modifying code, the branch modifying functions
57 * must be considered absolute slow paths (machine wide synchronization etc.).
58 * OTOH, since the affected branches are unconditional, their runtime overhead
59 * will be absolutely minimal, esp. in the default (off) case where the total
60 * effect is a single NOP of appropriate size. The on case will patch in a jump
61 * to the out-of-line block.
62 *
63 * When the control is directly exposed to userspace, it is prudent to delay the
64 * decrement to avoid high frequency code modifications which can (and do)
65 * cause significant performance degradation. Struct static_key_deferred and
66 * static_key_slow_dec_deferred() provide for this.
67 *
68 * Lacking toolchain and or architecture support, static keys fall back to a
69 * simple conditional branch.
70 *
71 * Additional babbling in: Documentation/static-keys.txt
72 */
73
74 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
75 # define HAVE_JUMP_LABEL
76 #endif
77
78 #ifndef __ASSEMBLY__
79
80 #include <linux/types.h>
81 #include <linux/compiler.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 /*
94 * Note:
95 * To make anonymous unions work with old compilers, the static
96 * initialization of them requires brackets. This creates a dependency
97 * on the order of the struct with the initializers. If any fields
98 * are added, STATIC_KEY_INIT_TRUE and STATIC_KEY_INIT_FALSE may need
99 * to be modified.
100 *
101 * bit 0 => 1 if key is initially true
102 * 0 if initially false
103 * bit 1 => 1 if points to struct static_key_mod
104 * 0 if points to struct jump_entry
105 */
106 union {
107 unsigned long type;
108 struct jump_entry *entries;
109 struct static_key_mod *next;
110 };
111 };
112
113 #else
114 struct static_key {
115 atomic_t enabled;
116 };
117 #endif /* HAVE_JUMP_LABEL */
118 #endif /* __ASSEMBLY__ */
119
120 #ifdef HAVE_JUMP_LABEL
121 #include <asm/jump_label.h>
122 #endif
123
124 #ifndef __ASSEMBLY__
125
126 enum jump_label_type {
127 JUMP_LABEL_NOP = 0,
128 JUMP_LABEL_JMP,
129 };
130
131 struct module;
132
133 #ifdef HAVE_JUMP_LABEL
134
135 #define JUMP_TYPE_FALSE 0UL
136 #define JUMP_TYPE_TRUE 1UL
137 #define JUMP_TYPE_LINKED 2UL
138 #define JUMP_TYPE_MASK 3UL
139
static_key_false(struct static_key * key)140 static __always_inline bool static_key_false(struct static_key *key)
141 {
142 return arch_static_branch(key, false);
143 }
144
static_key_true(struct static_key * key)145 static __always_inline bool static_key_true(struct static_key *key)
146 {
147 return !arch_static_branch(key, true);
148 }
149
150 extern struct jump_entry __start___jump_table[];
151 extern struct jump_entry __stop___jump_table[];
152
153 extern void jump_label_init(void);
154 extern void jump_label_lock(void);
155 extern void jump_label_unlock(void);
156 extern void arch_jump_label_transform(struct jump_entry *entry,
157 enum jump_label_type type);
158 extern void arch_jump_label_transform_static(struct jump_entry *entry,
159 enum jump_label_type type);
160 extern int jump_label_text_reserved(void *start, void *end);
161 extern void static_key_slow_inc(struct static_key *key);
162 extern void static_key_slow_dec(struct static_key *key);
163 extern void static_key_slow_inc_cpuslocked(struct static_key *key);
164 extern void static_key_slow_dec_cpuslocked(struct static_key *key);
165 extern void jump_label_apply_nops(struct module *mod);
166 extern int static_key_count(struct static_key *key);
167 extern void static_key_enable(struct static_key *key);
168 extern void static_key_disable(struct static_key *key);
169 extern void static_key_enable_cpuslocked(struct static_key *key);
170 extern void static_key_disable_cpuslocked(struct static_key *key);
171
172 /*
173 * We should be using ATOMIC_INIT() for initializing .enabled, but
174 * the inclusion of atomic.h is problematic for inclusion of jump_label.h
175 * in 'low-level' headers. Thus, we are initializing .enabled with a
176 * raw value, but have added a BUILD_BUG_ON() to catch any issues in
177 * jump_label_init() see: kernel/jump_label.c.
178 */
179 #define STATIC_KEY_INIT_TRUE \
180 { .enabled = { 1 }, \
181 { .entries = (void *)JUMP_TYPE_TRUE } }
182 #define STATIC_KEY_INIT_FALSE \
183 { .enabled = { 0 }, \
184 { .entries = (void *)JUMP_TYPE_FALSE } }
185
186 #else /* !HAVE_JUMP_LABEL */
187
188 #include <linux/atomic.h>
189 #include <linux/bug.h>
190
static_key_count(struct static_key * key)191 static inline int static_key_count(struct static_key *key)
192 {
193 return atomic_read(&key->enabled);
194 }
195
jump_label_init(void)196 static __always_inline void jump_label_init(void)
197 {
198 static_key_initialized = true;
199 }
200
static_key_false(struct static_key * key)201 static __always_inline bool static_key_false(struct static_key *key)
202 {
203 if (unlikely(static_key_count(key) > 0))
204 return true;
205 return false;
206 }
207
static_key_true(struct static_key * key)208 static __always_inline bool static_key_true(struct static_key *key)
209 {
210 if (likely(static_key_count(key) > 0))
211 return true;
212 return false;
213 }
214
static_key_slow_inc(struct static_key * key)215 static inline void static_key_slow_inc(struct static_key *key)
216 {
217 STATIC_KEY_CHECK_USE();
218 atomic_inc(&key->enabled);
219 }
220
static_key_slow_dec(struct static_key * key)221 static inline void static_key_slow_dec(struct static_key *key)
222 {
223 STATIC_KEY_CHECK_USE();
224 atomic_dec(&key->enabled);
225 }
226
227 #define static_key_slow_inc_cpuslocked(key) static_key_slow_inc(key)
228 #define static_key_slow_dec_cpuslocked(key) static_key_slow_dec(key)
229
jump_label_text_reserved(void * start,void * end)230 static inline int jump_label_text_reserved(void *start, void *end)
231 {
232 return 0;
233 }
234
jump_label_lock(void)235 static inline void jump_label_lock(void) {}
jump_label_unlock(void)236 static inline void jump_label_unlock(void) {}
237
jump_label_apply_nops(struct module * mod)238 static inline int jump_label_apply_nops(struct module *mod)
239 {
240 return 0;
241 }
242
static_key_enable(struct static_key * key)243 static inline void static_key_enable(struct static_key *key)
244 {
245 STATIC_KEY_CHECK_USE();
246
247 if (atomic_read(&key->enabled) != 0) {
248 WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
249 return;
250 }
251 atomic_set(&key->enabled, 1);
252 }
253
static_key_disable(struct static_key * key)254 static inline void static_key_disable(struct static_key *key)
255 {
256 STATIC_KEY_CHECK_USE();
257
258 if (atomic_read(&key->enabled) != 1) {
259 WARN_ON_ONCE(atomic_read(&key->enabled) != 0);
260 return;
261 }
262 atomic_set(&key->enabled, 0);
263 }
264
265 #define static_key_enable_cpuslocked(k) static_key_enable((k))
266 #define static_key_disable_cpuslocked(k) static_key_disable((k))
267
268 #define STATIC_KEY_INIT_TRUE { .enabled = ATOMIC_INIT(1) }
269 #define STATIC_KEY_INIT_FALSE { .enabled = ATOMIC_INIT(0) }
270
271 #endif /* HAVE_JUMP_LABEL */
272
273 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
274 #define jump_label_enabled static_key_enabled
275
276 /* -------------------------------------------------------------------------- */
277
278 /*
279 * Two type wrappers around static_key, such that we can use compile time
280 * type differentiation to emit the right code.
281 *
282 * All the below code is macros in order to play type games.
283 */
284
285 struct static_key_true {
286 struct static_key key;
287 };
288
289 struct static_key_false {
290 struct static_key key;
291 };
292
293 #define STATIC_KEY_TRUE_INIT (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE, }
294 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
295
296 #define DEFINE_STATIC_KEY_TRUE(name) \
297 struct static_key_true name = STATIC_KEY_TRUE_INIT
298
299 #define DECLARE_STATIC_KEY_TRUE(name) \
300 extern struct static_key_true name
301
302 #define DEFINE_STATIC_KEY_FALSE(name) \
303 struct static_key_false name = STATIC_KEY_FALSE_INIT
304
305 #define DECLARE_STATIC_KEY_FALSE(name) \
306 extern struct static_key_false name
307
308 #define DEFINE_STATIC_KEY_ARRAY_TRUE(name, count) \
309 struct static_key_true name[count] = { \
310 [0 ... (count) - 1] = STATIC_KEY_TRUE_INIT, \
311 }
312
313 #define DEFINE_STATIC_KEY_ARRAY_FALSE(name, count) \
314 struct static_key_false name[count] = { \
315 [0 ... (count) - 1] = STATIC_KEY_FALSE_INIT, \
316 }
317
318 extern bool ____wrong_branch_error(void);
319
320 #define static_key_enabled(x) \
321 ({ \
322 if (!__builtin_types_compatible_p(typeof(*x), struct static_key) && \
323 !__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
324 !__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
325 ____wrong_branch_error(); \
326 static_key_count((struct static_key *)x) > 0; \
327 })
328
329 #ifdef HAVE_JUMP_LABEL
330
331 /*
332 * Combine the right initial value (type) with the right branch order
333 * to generate the desired result.
334 *
335 *
336 * type\branch| likely (1) | unlikely (0)
337 * -----------+-----------------------+------------------
338 * | |
339 * true (1) | ... | ...
340 * | NOP | JMP L
341 * | <br-stmts> | 1: ...
342 * | L: ... |
343 * | |
344 * | | L: <br-stmts>
345 * | | jmp 1b
346 * | |
347 * -----------+-----------------------+------------------
348 * | |
349 * false (0) | ... | ...
350 * | JMP L | NOP
351 * | <br-stmts> | 1: ...
352 * | L: ... |
353 * | |
354 * | | L: <br-stmts>
355 * | | jmp 1b
356 * | |
357 * -----------+-----------------------+------------------
358 *
359 * The initial value is encoded in the LSB of static_key::entries,
360 * type: 0 = false, 1 = true.
361 *
362 * The branch type is encoded in the LSB of jump_entry::key,
363 * branch: 0 = unlikely, 1 = likely.
364 *
365 * This gives the following logic table:
366 *
367 * enabled type branch instuction
368 * -----------------------------+-----------
369 * 0 0 0 | NOP
370 * 0 0 1 | JMP
371 * 0 1 0 | NOP
372 * 0 1 1 | JMP
373 *
374 * 1 0 0 | JMP
375 * 1 0 1 | NOP
376 * 1 1 0 | JMP
377 * 1 1 1 | NOP
378 *
379 * Which gives the following functions:
380 *
381 * dynamic: instruction = enabled ^ branch
382 * static: instruction = type ^ branch
383 *
384 * See jump_label_type() / jump_label_init_type().
385 */
386
387 #define static_branch_likely(x) \
388 ({ \
389 bool branch; \
390 if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
391 branch = !arch_static_branch(&(x)->key, true); \
392 else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
393 branch = !arch_static_branch_jump(&(x)->key, true); \
394 else \
395 branch = ____wrong_branch_error(); \
396 branch; \
397 })
398
399 #define static_branch_unlikely(x) \
400 ({ \
401 bool branch; \
402 if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
403 branch = arch_static_branch_jump(&(x)->key, false); \
404 else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
405 branch = arch_static_branch(&(x)->key, false); \
406 else \
407 branch = ____wrong_branch_error(); \
408 branch; \
409 })
410
411 #else /* !HAVE_JUMP_LABEL */
412
413 #define static_branch_likely(x) likely(static_key_enabled(&(x)->key))
414 #define static_branch_unlikely(x) unlikely(static_key_enabled(&(x)->key))
415
416 #endif /* HAVE_JUMP_LABEL */
417
418 /*
419 * Advanced usage; refcount, branch is enabled when: count != 0
420 */
421
422 #define static_branch_inc(x) static_key_slow_inc(&(x)->key)
423 #define static_branch_dec(x) static_key_slow_dec(&(x)->key)
424 #define static_branch_inc_cpuslocked(x) static_key_slow_inc_cpuslocked(&(x)->key)
425 #define static_branch_dec_cpuslocked(x) static_key_slow_dec_cpuslocked(&(x)->key)
426
427 /*
428 * Normal usage; boolean enable/disable.
429 */
430
431 #define static_branch_enable(x) static_key_enable(&(x)->key)
432 #define static_branch_disable(x) static_key_disable(&(x)->key)
433 #define static_branch_enable_cpuslocked(x) static_key_enable_cpuslocked(&(x)->key)
434 #define static_branch_disable_cpuslocked(x) static_key_disable_cpuslocked(&(x)->key)
435
436 #endif /* __ASSEMBLY__ */
437
438 #endif /* _LINUX_JUMP_LABEL_H */
439