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 * Jump labels provide an interface to generate dynamic branches using 11 * self-modifying code. Assuming toolchain and architecture support the result 12 * of a "if (static_key_false(&key))" statement is a unconditional branch (which 13 * defaults to false - and the true block is placed out of line). 14 * 15 * However at runtime we can change the branch target using 16 * static_key_slow_{inc,dec}(). These function as a 'reference' count on the key 17 * object and for as long as there are references all branches referring to 18 * that particular key will point to the (out of line) true block. 19 * 20 * Since this relies on modifying code the static_key_slow_{inc,dec}() functions 21 * must be considered absolute slow paths (machine wide synchronization etc.). 22 * OTOH, since the affected branches are unconditional their runtime overhead 23 * will be absolutely minimal, esp. in the default (off) case where the total 24 * effect is a single NOP of appropriate size. The on case will patch in a jump 25 * to the out-of-line block. 26 * 27 * When the control is directly exposed to userspace it is prudent to delay the 28 * decrement to avoid high frequency code modifications which can (and do) 29 * cause significant performance degradation. Struct static_key_deferred and 30 * static_key_slow_dec_deferred() provide for this. 31 * 32 * Lacking toolchain and or architecture support, it falls back to a simple 33 * conditional branch. 34 * 35 * struct static_key my_key = STATIC_KEY_INIT_TRUE; 36 * 37 * if (static_key_true(&my_key)) { 38 * } 39 * 40 * will result in the true case being in-line and starts the key with a single 41 * reference. Mixing static_key_true() and static_key_false() on the same key is not 42 * allowed. 43 * 44 * Not initializing the key (static data is initialized to 0s anyway) is the 45 * same as using STATIC_KEY_INIT_FALSE. 46 * 47 */ 48 49 #include <linux/types.h> 50 #include <linux/compiler.h> 51 52 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL) 53 54 struct static_key { 55 atomic_t enabled; 56 /* Set lsb bit to 1 if branch is default true, 0 ot */ 57 struct jump_entry *entries; 58 #ifdef CONFIG_MODULES 59 struct static_key_mod *next; 60 #endif 61 }; 62 63 # include <asm/jump_label.h> 64 # define HAVE_JUMP_LABEL 65 #endif /* CC_HAVE_ASM_GOTO && CONFIG_JUMP_LABEL */ 66 67 enum jump_label_type { 68 JUMP_LABEL_DISABLE = 0, 69 JUMP_LABEL_ENABLE, 70 }; 71 72 struct module; 73 74 #include <linux/atomic.h> 75 #ifdef HAVE_JUMP_LABEL 76 77 #define JUMP_LABEL_TRUE_BRANCH 1UL 78 79 static 80 inline struct jump_entry *jump_label_get_entries(struct static_key *key) 81 { 82 return (struct jump_entry *)((unsigned long)key->entries 83 & ~JUMP_LABEL_TRUE_BRANCH); 84 } 85 86 static inline bool jump_label_get_branch_default(struct static_key *key) 87 { 88 if ((unsigned long)key->entries & JUMP_LABEL_TRUE_BRANCH) 89 return true; 90 return false; 91 } 92 93 static __always_inline bool static_key_false(struct static_key *key) 94 { 95 return arch_static_branch(key); 96 } 97 98 static __always_inline bool static_key_true(struct static_key *key) 99 { 100 return !static_key_false(key); 101 } 102 103 extern struct jump_entry __start___jump_table[]; 104 extern struct jump_entry __stop___jump_table[]; 105 106 extern void jump_label_init(void); 107 extern void jump_label_lock(void); 108 extern void jump_label_unlock(void); 109 extern void arch_jump_label_transform(struct jump_entry *entry, 110 enum jump_label_type type); 111 extern void arch_jump_label_transform_static(struct jump_entry *entry, 112 enum jump_label_type type); 113 extern int jump_label_text_reserved(void *start, void *end); 114 extern void static_key_slow_inc(struct static_key *key); 115 extern void static_key_slow_dec(struct static_key *key); 116 extern void jump_label_apply_nops(struct module *mod); 117 118 #define STATIC_KEY_INIT_TRUE ((struct static_key) \ 119 { .enabled = ATOMIC_INIT(1), .entries = (void *)1 }) 120 #define STATIC_KEY_INIT_FALSE ((struct static_key) \ 121 { .enabled = ATOMIC_INIT(0), .entries = (void *)0 }) 122 123 #else /* !HAVE_JUMP_LABEL */ 124 125 struct static_key { 126 atomic_t enabled; 127 }; 128 129 static __always_inline void jump_label_init(void) 130 { 131 } 132 133 static __always_inline bool static_key_false(struct static_key *key) 134 { 135 if (unlikely(atomic_read(&key->enabled)) > 0) 136 return true; 137 return false; 138 } 139 140 static __always_inline bool static_key_true(struct static_key *key) 141 { 142 if (likely(atomic_read(&key->enabled)) > 0) 143 return true; 144 return false; 145 } 146 147 static inline void static_key_slow_inc(struct static_key *key) 148 { 149 atomic_inc(&key->enabled); 150 } 151 152 static inline void static_key_slow_dec(struct static_key *key) 153 { 154 atomic_dec(&key->enabled); 155 } 156 157 static inline int jump_label_text_reserved(void *start, void *end) 158 { 159 return 0; 160 } 161 162 static inline void jump_label_lock(void) {} 163 static inline void jump_label_unlock(void) {} 164 165 static inline int jump_label_apply_nops(struct module *mod) 166 { 167 return 0; 168 } 169 170 #define STATIC_KEY_INIT_TRUE ((struct static_key) \ 171 { .enabled = ATOMIC_INIT(1) }) 172 #define STATIC_KEY_INIT_FALSE ((struct static_key) \ 173 { .enabled = ATOMIC_INIT(0) }) 174 175 #endif /* HAVE_JUMP_LABEL */ 176 177 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE 178 #define jump_label_enabled static_key_enabled 179 180 static inline bool static_key_enabled(struct static_key *key) 181 { 182 return (atomic_read(&key->enabled) > 0); 183 } 184 185 #endif /* _LINUX_JUMP_LABEL_H */ 186