On Tue, Aug 30, 2022 at 12:44:13PM +0200, Peter Zijlstra wrote: > On Fri, Aug 26, 2022 at 08:46:08PM +0200, Jiri Olsa wrote: > > The dispatcher function is attached/detached to trampoline by > > dispatcher update function. At the same time it's available as > > ftrace attachable function. > > > > After discussion [1] the proposed solution is to use compiler > > attributes to alter bpf_dispatcher_##name##_func function: > > > > - remove it from being instrumented with __no_instrument_function__ > > attribute, so ftrace has no track of it > > This is typically spelled like: 'notrace' in the kernel. > > > - but still generate 5 nop instructions with patchable_function_entry(5) > > attribute, which are expected by bpf_arch_text_poke used by > > dispatcher update function > > > > Enabling HAVE_DYNAMIC_FTRACE_NO_PATCHABLE option for x86, so > > __patchable_function_entries functions are not part of ftrace/mcount > > locations. > > > > The dispatcher code is generated and attached only for x86 so it's safe > > to keep bpf_dispatcher func in patchable_function_entry locations for > > other archs. > > > > [1] https://lore.kernel.org/bpf/20220722110811.124515-1-jolsa@xxxxxxxxxx/ > > Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx> > > --- > > arch/x86/Kconfig | 1 + > > include/linux/bpf.h | 2 ++ > > 2 files changed, 3 insertions(+) > > > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig > > index f9920f1341c8..089c20cefd2b 100644 > > --- a/arch/x86/Kconfig > > +++ b/arch/x86/Kconfig > > @@ -284,6 +284,7 @@ config X86 > > select PROC_PID_ARCH_STATUS if PROC_FS > > select HAVE_ARCH_NODE_DEV_GROUP if X86_SGX > > imply IMA_SECURE_AND_OR_TRUSTED_BOOT if EFI > > + select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE > > > > config INSTRUCTION_DECODER > > def_bool y > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h > > index 9c1674973e03..945d5414bb62 100644 > > --- a/include/linux/bpf.h > > +++ b/include/linux/bpf.h > > @@ -925,6 +925,8 @@ int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs); > > } > > > > #define DEFINE_BPF_DISPATCHER(name) \ > > + __attribute__((__no_instrument_function__)) \ > > + __attribute__((patchable_function_entry(5))) \ > > noinline __nocfi unsigned int bpf_dispatcher_##name##_func( \ > > const void *ctx, \ > > const struct bpf_insn *insnsi, \ > > What makes that whole dispatcher thing x86 only? AFAICT it is only under > BPF_JIT here and could be used by anyone. it is just optimalization on x86, that transform default indirect calls to direct call, described in changelog in here: 75ccbef6369e bpf: Introduce BPF dispatcher other archs just make the call to bpf_func jirka > > ARM64 for instance has BPG_JIT and builds net/core/filter.c. And ARM64 > very much does use patchable_function_entry() for its ftrace > implementation.
