Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes: [...] > > +Helper functions > > +~~~~~~~~~~~~~~~~ > > + > > +Helper functions are a concept whereby BPF programs can call into a > > +set of function calls exposed by the runtime. Each helper function > > +is identified by an integer used in a ``BPF_CALL`` instruction. > > +The available helper functions may differ for each program type. > > + > > +Conceptually, each helper function is implemented with a commonly > > +shared function signature defined as: > > + > > + u64 function(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5) > > + > > +In actuality, each helper function is defined as taking between 0 and > > +5 arguments, with the remaining registers being ignored. The > > +definition of a helper function is responsible for specifying the > > +type (e.g., integer, pointer, etc.) of the value returned, the number of > arguments, and the type of each argument. > > Above is correct, but it aims to describe the calling convention which should > be done in a separate BPF psABI doc and not in instruction-set.rst. > And if we start describing calling convention we should talk about promotion > rules, sign extensions, expectations for return values, for passing structs by > value, etc. The instruction itself requires defining the concept of a helper function, so is the text in question the part starting with "Conceptually," down to the end of the quoted text? Since there is no separate BPF psABI document (and I'm not sure the scope of that document myself) can we put it here for now and move it when that doc is created? If not, what part of the text above would be in a separate document? [...] Dave
