On Tue, Sep 27, 2022 at 06:59:52PM +0000, dthaler1968@xxxxxxxxxxxxxx wrote: > From: Dave Thaler <dthaler@xxxxxxxxxxxxx> > > Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> > --- > Documentation/bpf/instruction-set.rst | 18 +++++++++++++++++- > 1 file changed, 17 insertions(+), 1 deletion(-) > > diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst > index 2987234eb..926957830 100644 > --- a/Documentation/bpf/instruction-set.rst > +++ b/Documentation/bpf/instruction-set.rst > @@ -245,7 +245,7 @@ BPF_JSET 0x40 PC += off if dst & src > BPF_JNE 0x50 PC += off if dst != src > BPF_JSGT 0x60 PC += off if dst > src signed > BPF_JSGE 0x70 PC += off if dst >= src signed > -BPF_CALL 0x80 function call > +BPF_CALL 0x80 function call see `Helper functions`_ > BPF_EXIT 0x90 function / program return BPF_JMP only > BPF_JLT 0xa0 PC += off if dst < src unsigned > BPF_JLE 0xb0 PC += off if dst <= src unsigned > @@ -256,6 +256,22 @@ BPF_JSLE 0xd0 PC += off if dst <= src signed > The eBPF program needs to store the return value into register R0 before doing a > BPF_EXIT. > > +Helper functions > +~~~~~~~~~~~~~~~~ > +Helper functions are a concept whereby BPF programs can call into a > +set of function calls exposed by the eBPF runtime. Each helper > +function is identified by an integer used in a ``BPF_CALL`` instruction. > +The available helper functions may differ for each eBPF program type. > + > +Conceptually, each helper function is implemented with a commonly shared function > +signature defined as: > + > + uint64_t function(uint64_t r1, uint64_t r2, uint64_t r3, uint64_t r4, uint64_t 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. If we explain helpers in the doc then we should explain kfuncs and bpf-to-bpf calls as well. Otherwise it looks incomplete and eventually will suffer the same issue as '64-bit instructionS'. Here it's only one CALL insn. Though 'imm' value can be interpreted differently bpf2bpf vs helper vs kfunc.
