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Diffstat (limited to 'include/linux/static_call.h')
| -rw-r--r-- | include/linux/static_call.h | 298 |
1 files changed, 298 insertions, 0 deletions
diff --git a/include/linux/static_call.h b/include/linux/static_call.h new file mode 100644 index 000000000000..695da4c9b338 --- /dev/null +++ b/include/linux/static_call.h @@ -0,0 +1,298 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _LINUX_STATIC_CALL_H +#define _LINUX_STATIC_CALL_H + +/* + * Static call support + * + * Static calls use code patching to hard-code function pointers into direct + * branch instructions. They give the flexibility of function pointers, but + * with improved performance. This is especially important for cases where + * retpolines would otherwise be used, as retpolines can significantly impact + * performance. + * + * + * API overview: + * + * DECLARE_STATIC_CALL(name, func); + * DEFINE_STATIC_CALL(name, func); + * DEFINE_STATIC_CALL_NULL(name, typename); + * static_call(name)(args...); + * static_call_cond(name)(args...); + * static_call_update(name, func); + * + * Usage example: + * + * # Start with the following functions (with identical prototypes): + * int func_a(int arg1, int arg2); + * int func_b(int arg1, int arg2); + * + * # Define a 'my_name' reference, associated with func_a() by default + * DEFINE_STATIC_CALL(my_name, func_a); + * + * # Call func_a() + * static_call(my_name)(arg1, arg2); + * + * # Update 'my_name' to point to func_b() + * static_call_update(my_name, &func_b); + * + * # Call func_b() + * static_call(my_name)(arg1, arg2); + * + * + * Implementation details: + * + * This requires some arch-specific code (CONFIG_HAVE_STATIC_CALL). + * Otherwise basic indirect calls are used (with function pointers). + * + * Each static_call() site calls into a trampoline associated with the name. + * The trampoline has a direct branch to the default function. Updates to a + * name will modify the trampoline's branch destination. + * + * If the arch has CONFIG_HAVE_STATIC_CALL_INLINE, then the call sites + * themselves will be patched at runtime to call the functions directly, + * rather than calling through the trampoline. This requires objtool or a + * compiler plugin to detect all the static_call() sites and annotate them + * in the .static_call_sites section. + * + * + * Notes on NULL function pointers: + * + * Static_call()s support NULL functions, with many of the caveats that + * regular function pointers have. + * + * Clearly calling a NULL function pointer is 'BAD', so too for + * static_call()s (although when HAVE_STATIC_CALL it might not be immediately + * fatal). A NULL static_call can be the result of: + * + * DECLARE_STATIC_CALL_NULL(my_static_call, void (*)(int)); + * + * which is equivalent to declaring a NULL function pointer with just a + * typename: + * + * void (*my_func_ptr)(int arg1) = NULL; + * + * or using static_call_update() with a NULL function. In both cases the + * HAVE_STATIC_CALL implementation will patch the trampoline with a RET + * instruction, instead of an immediate tail-call JMP. HAVE_STATIC_CALL_INLINE + * architectures can patch the trampoline call to a NOP. + * + * In all cases, any argument evaluation is unconditional. Unlike a regular + * conditional function pointer call: + * + * if (my_func_ptr) + * my_func_ptr(arg1) + * + * where the argument evaludation also depends on the pointer value. + * + * When calling a static_call that can be NULL, use: + * + * static_call_cond(name)(arg1); + * + * which will include the required value tests to avoid NULL-pointer + * dereferences. + */ + +#include <linux/types.h> +#include <linux/cpu.h> +#include <linux/static_call_types.h> + +#ifdef CONFIG_HAVE_STATIC_CALL +#include <asm/static_call.h> + +/* + * Either @site or @tramp can be NULL. + */ +extern void arch_static_call_transform(void *site, void *tramp, void *func, bool tail); + +#define STATIC_CALL_TRAMP_ADDR(name) &STATIC_CALL_TRAMP(name) + +/* + * __ADDRESSABLE() is used to ensure the key symbol doesn't get stripped from + * the symbol table so that objtool can reference it when it generates the + * .static_call_sites section. + */ +#define __static_call(name) \ +({ \ + __ADDRESSABLE(STATIC_CALL_KEY(name)); \ + &STATIC_CALL_TRAMP(name); \ +}) + +#else +#define STATIC_CALL_TRAMP_ADDR(name) NULL +#endif + + +#define DECLARE_STATIC_CALL(name, func) \ + extern struct static_call_key STATIC_CALL_KEY(name); \ + extern typeof(func) STATIC_CALL_TRAMP(name); + +#define static_call_update(name, func) \ +({ \ + BUILD_BUG_ON(!__same_type(*(func), STATIC_CALL_TRAMP(name))); \ + __static_call_update(&STATIC_CALL_KEY(name), \ + STATIC_CALL_TRAMP_ADDR(name), func); \ +}) + +#ifdef CONFIG_HAVE_STATIC_CALL_INLINE + +extern int __init static_call_init(void); + +struct static_call_mod { + struct static_call_mod *next; + struct module *mod; /* for vmlinux, mod == NULL */ + struct static_call_site *sites; +}; + +struct static_call_key { + void *func; + union { + /* bit 0: 0 = mods, 1 = sites */ + unsigned long type; + struct static_call_mod *mods; + struct static_call_site *sites; + }; +}; + +extern void __static_call_update(struct static_call_key *key, void *tramp, void *func); +extern int static_call_mod_init(struct module *mod); +extern int static_call_text_reserved(void *start, void *end); + +#define DEFINE_STATIC_CALL(name, _func) \ + DECLARE_STATIC_CALL(name, _func); \ + struct static_call_key STATIC_CALL_KEY(name) = { \ + .func = _func, \ + .type = 1, \ + }; \ + ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func) + +#define DEFINE_STATIC_CALL_NULL(name, _func) \ + DECLARE_STATIC_CALL(name, _func); \ + struct static_call_key STATIC_CALL_KEY(name) = { \ + .func = NULL, \ + .type = 1, \ + }; \ + ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) + +#define static_call(name) __static_call(name) +#define static_call_cond(name) (void)__static_call(name) + +#define EXPORT_STATIC_CALL(name) \ + EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \ + EXPORT_SYMBOL(STATIC_CALL_TRAMP(name)) + +#define EXPORT_STATIC_CALL_GPL(name) \ + EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \ + EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name)) + +#elif defined(CONFIG_HAVE_STATIC_CALL) + +static inline int static_call_init(void) { return 0; } + +struct static_call_key { + void *func; +}; + +#define DEFINE_STATIC_CALL(name, _func) \ + DECLARE_STATIC_CALL(name, _func); \ + struct static_call_key STATIC_CALL_KEY(name) = { \ + .func = _func, \ + }; \ + ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func) + +#define DEFINE_STATIC_CALL_NULL(name, _func) \ + DECLARE_STATIC_CALL(name, _func); \ + struct static_call_key STATIC_CALL_KEY(name) = { \ + .func = NULL, \ + }; \ + ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) + +#define static_call(name) __static_call(name) +#define static_call_cond(name) (void)__static_call(name) + +static inline +void __static_call_update(struct static_call_key *key, void *tramp, void *func) +{ + cpus_read_lock(); + WRITE_ONCE(key->func, func); + arch_static_call_transform(NULL, tramp, func, false); + cpus_read_unlock(); +} + +static inline int static_call_text_reserved(void *start, void *end) +{ + return 0; +} + +#define EXPORT_STATIC_CALL(name) \ + EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \ + EXPORT_SYMBOL(STATIC_CALL_TRAMP(name)) + +#define EXPORT_STATIC_CALL_GPL(name) \ + EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \ + EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name)) + +#else /* Generic implementation */ + +static inline int static_call_init(void) { return 0; } + +struct static_call_key { + void *func; +}; + +#define DEFINE_STATIC_CALL(name, _func) \ + DECLARE_STATIC_CALL(name, _func); \ + struct static_call_key STATIC_CALL_KEY(name) = { \ + .func = _func, \ + } + +#define DEFINE_STATIC_CALL_NULL(name, _func) \ + DECLARE_STATIC_CALL(name, _func); \ + struct static_call_key STATIC_CALL_KEY(name) = { \ + .func = NULL, \ + } + +#define static_call(name) \ + ((typeof(STATIC_CALL_TRAMP(name))*)(STATIC_CALL_KEY(name).func)) + +static inline void __static_call_nop(void) { } + +/* + * This horrific hack takes care of two things: + * + * - it ensures the compiler will only load the function pointer ONCE, + * which avoids a reload race. + * + * - it ensures the argument evaluation is unconditional, similar + * to the HAVE_STATIC_CALL variant. + * + * Sadly current GCC/Clang (10 for both) do not optimize this properly + * and will emit an indirect call for the NULL case :-( + */ +#define __static_call_cond(name) \ +({ \ + void *func = READ_ONCE(STATIC_CALL_KEY(name).func); \ + if (!func) \ + func = &__static_call_nop; \ + (typeof(STATIC_CALL_TRAMP(name))*)func; \ +}) + +#define static_call_cond(name) (void)__static_call_cond(name) + +static inline +void __static_call_update(struct static_call_key *key, void *tramp, void *func) +{ + WRITE_ONCE(key->func, func); +} + +static inline int static_call_text_reserved(void *start, void *end) +{ + return 0; +} + +#define EXPORT_STATIC_CALL(name) EXPORT_SYMBOL(STATIC_CALL_KEY(name)) +#define EXPORT_STATIC_CALL_GPL(name) EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)) + +#endif /* CONFIG_HAVE_STATIC_CALL */ + +#endif /* _LINUX_STATIC_CALL_H */ |