The following commit has been merged into the core/static_call branch of tip:
Commit-ID: 452cddbff74b6a15b9354505671011700fe03710 Gitweb: https://git.kernel.org/tip/452cddbff74b6a15b9354505671011700fe03710 Author: Peter Zijlstra <pet...@infradead.org> AuthorDate: Tue, 18 Aug 2020 15:57:48 +02:00 Committer: Ingo Molnar <mi...@kernel.org> CommitterDate: Tue, 01 Sep 2020 09:58:05 +02:00 static_call: Add static_call_cond() Extend the static_call infrastructure to optimize the following common pattern: if (func_ptr) func_ptr(args...) For the trampoline (which is in effect a tail-call), we patch the JMP.d32 into a RET, which then directly consumes the trampoline call. For the in-line sites we replace the CALL with a NOP5. NOTE: this is 'obviously' limited to functions with a 'void' return type. NOTE: DEFINE_STATIC_COND_CALL() only requires a typename, as opposed to a full function. Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Signed-off-by: Ingo Molnar <mi...@kernel.org> Cc: Linus Torvalds <torva...@linux-foundation.org> Link: https://lore.kernel.org/r/20200818135805.042977...@infradead.org --- arch/x86/include/asm/static_call.h | 12 +++- arch/x86/kernel/static_call.c | 42 ++++++++++---- include/linux/static_call.h | 86 +++++++++++++++++++++++++++++- 3 files changed, 127 insertions(+), 13 deletions(-) diff --git a/arch/x86/include/asm/static_call.h b/arch/x86/include/asm/static_call.h index 33469ae..c37f119 100644 --- a/arch/x86/include/asm/static_call.h +++ b/arch/x86/include/asm/static_call.h @@ -20,15 +20,21 @@ * it does tail-call optimization on the call; since you cannot compute the * relative displacement across sections. */ -#define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func) \ + +#define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, insns) \ asm(".pushsection .static_call.text, \"ax\" \n" \ ".align 4 \n" \ ".globl " STATIC_CALL_TRAMP_STR(name) " \n" \ STATIC_CALL_TRAMP_STR(name) ": \n" \ - " .byte 0xe9 # jmp.d32 \n" \ - " .long " #func " - (. + 4) \n" \ + insns " \n" \ ".type " STATIC_CALL_TRAMP_STR(name) ", @function \n" \ ".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \ ".popsection \n") +#define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func) \ + __ARCH_DEFINE_STATIC_CALL_TRAMP(name, ".byte 0xe9; .long " #func " - (. + 4)") + +#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \ + __ARCH_DEFINE_STATIC_CALL_TRAMP(name, "ret; nop; nop; nop; nop") + #endif /* _ASM_STATIC_CALL_H */ diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c index 5ff2b63..ead6726 100644 --- a/arch/x86/kernel/static_call.c +++ b/arch/x86/kernel/static_call.c @@ -4,19 +4,41 @@ #include <linux/bug.h> #include <asm/text-patching.h> -static void __static_call_transform(void *insn, u8 opcode, void *func) +enum insn_type { + CALL = 0, /* site call */ + NOP = 1, /* site cond-call */ + JMP = 2, /* tramp / site tail-call */ + RET = 3, /* tramp / site cond-tail-call */ +}; + +static void __static_call_transform(void *insn, enum insn_type type, void *func) { - const void *code = text_gen_insn(opcode, insn, func); + int size = CALL_INSN_SIZE; + const void *code; - if (WARN_ONCE(*(u8 *)insn != opcode, - "unexpected static call insn opcode 0x%x at %pS\n", - opcode, insn)) - return; + switch (type) { + case CALL: + code = text_gen_insn(CALL_INSN_OPCODE, insn, func); + break; + + case NOP: + code = ideal_nops[NOP_ATOMIC5]; + break; + + case JMP: + code = text_gen_insn(JMP32_INSN_OPCODE, insn, func); + break; + + case RET: + code = text_gen_insn(RET_INSN_OPCODE, insn, func); + size = RET_INSN_SIZE; + break; + } - if (memcmp(insn, code, CALL_INSN_SIZE) == 0) + if (memcmp(insn, code, size) == 0) return; - text_poke_bp(insn, code, CALL_INSN_SIZE, NULL); + text_poke_bp(insn, code, size, NULL); } void arch_static_call_transform(void *site, void *tramp, void *func) @@ -24,10 +46,10 @@ void arch_static_call_transform(void *site, void *tramp, void *func) mutex_lock(&text_mutex); if (tramp) - __static_call_transform(tramp, JMP32_INSN_OPCODE, func); + __static_call_transform(tramp, func ? JMP : RET, func); if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) - __static_call_transform(site, CALL_INSN_OPCODE, func); + __static_call_transform(site, func ? CALL : NOP, func); mutex_unlock(&text_mutex); } diff --git a/include/linux/static_call.h b/include/linux/static_call.h index 6f62ced..0f74581 100644 --- a/include/linux/static_call.h +++ b/include/linux/static_call.h @@ -16,7 +16,9 @@ * * 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: @@ -52,6 +54,43 @@ * 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> @@ -120,7 +159,16 @@ extern int static_call_text_reserved(void *start, void *end); }; \ 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)); \ @@ -143,7 +191,15 @@ struct static_call_key { }; \ 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) @@ -179,9 +235,39 @@ struct static_call_key { .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) {
