On March 4, 2025 5:19:09 PM PST, Menglong Dong <menglong8.dong@xxxxxxxxx> wrote: >On Tue, Mar 4, 2025 at 10:53 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote: >> >> On March 4, 2025 1:42:20 AM PST, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: >> >On Tue, Mar 04, 2025 at 03:47:45PM +0800, Menglong Dong wrote: >> >> We don't have to select FUNCTION_ALIGNMENT_32B, so the >> >> worst case is to increase ~2.2%. >> >> >> >> What do you think? >> > >> >Well, since I don't understand what you need this for at all, I'm firmly >> >on the side of not doing this. >> > >> >What actual problem is being solved with this meta data nonsense? Why is >> >it worth blowing up our I$ footprint over. >> > >> >Also note, that if you're going to be explaining this, start from >> >scratch, as I have absolutely 0 clues about BPF and such. >> >> I would appreciate such information as well. The idea seems dubious on the surface. > >Ok, let me explain it from the beginning. (My English is not good, >but I'll try to describe it as clear as possible :/) > >Many BPF program types need to depend on the BPF trampoline, >such as BPF_PROG_TYPE_TRACING, BPF_PROG_TYPE_EXT, >BPF_PROG_TYPE_LSM, etc. BPF trampoline is a bridge between >the kernel (or bpf) function and BPF program, and it acts just like the >trampoline that ftrace uses. > >Generally speaking, it is used to hook a function, just like what ftrace >do: > >foo: > endbr > nop5 --> call trampoline_foo > xxxx > >In short, the trampoline_foo can be this: > >trampoline_foo: > prepare a array and store the args of foo to the array > call fentry_bpf1 > call fentry_bpf2 > ...... > call foo+4 (origin call) > save the return value of foo > call fexit_bpf1 (this bpf can get the return value of foo) > call fexit_bpf2 > ....... > return to the caller of foo > >We can see that the trampoline_foo can be only used for >the function foo, as different kernel function can be attached >different BPF programs, and have different argument count, >etc. Therefore, we have to create 1000 BPF trampolines if >we want to attach a BPF program to 1000 kernel functions. > >The creation of the BPF trampoline is expensive. According to >my testing, It will spend more than 1 second to create 100 bpf >trampoline. What's more, it consumes more memory. > >If we have the per-function metadata supporting, then we can >create a global BPF trampoline, like this: > >trampoline_global: > prepare a array and store the args of foo to the array > get the metadata by the ip > call metadata.fentry_bpf1 > call metadata.fentry_bpf2 > .... > call foo+4 (origin call) > save the return value of foo > call metadata.fexit_bpf1 (this bpf can get the return value of foo) > call metadata.fexit_bpf2 > ....... > return to the caller of foo > >(The metadata holds more information for the global trampoline than >I described.) > >Then, we don't need to create a trampoline for every kernel function >anymore. > >Another beneficiary can be ftrace. For now, all the kernel functions that >are enabled by dynamic ftrace will be added to a filter hash if there are >more than one callbacks. And hash lookup will happen when the traced >functions are called, which has an impact on the performance, see >__ftrace_ops_list_func() -> ftrace_ops_test(). With the per-function >metadata supporting, we can store the information that if the callback is >enabled on the kernel function to the metadata, which can make the performance >much better. > >The per-function metadata storage is a basic function, and I think there >may be other functions that can use it for better performance in the feature >too. > >(Hope that I'm describing it clearly :/) > >Thanks! >Menglong Dong > This is way too cursory. For one thing, you need to start by explaining why you are asking to put this *inline* with the code, which is something that normally would be avoided at all cost.
