On Mon, Feb 06, 2023 at 07:16:47PM +0000, Dave Thaler wrote: > From: Dave Thaler <dthaler@xxxxxxxxxxxxx> > > Add text explaining helper functions. > Note that text about runtime functions (kfuncs) is part of a separate patch, > not this one. > > Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> > --- > V1 -> V2: addressed comments from Alexei and Stanislav > --- > Documentation/bpf/clang-notes.rst | 5 +++++ > Documentation/bpf/instruction-set.rst | 22 +++++++++++++++++++++- > 2 files changed, 26 insertions(+), 1 deletion(-) > > diff --git a/Documentation/bpf/clang-notes.rst b/Documentation/bpf/clang-notes.rst > index 528feddf2db..40c6185513a 100644 > --- a/Documentation/bpf/clang-notes.rst > +++ b/Documentation/bpf/clang-notes.rst > @@ -20,6 +20,11 @@ Arithmetic instructions > For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with > ``-Xclang -target-feature -Xclang +alu32``. In CPU version 3, support is automatically included. > > +Reserved instructions > +==================== small nit: Missing a = > + > +Clang will generate the reserved ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d) instruction if ``-O0`` is used. Are we calling this out here to say that BPF_CALL in clang -O0 builds is not supported? That would seem to be the case given that we say that BPF_CALL | BPF_X | BPF_JMP in reserved and not permitted in instruction-set.rst. If that's not the case, can we add a bit more verbiage here describing why this is done / why it's interesting and/or relevant to the reader? FWIW, most of our selftests don't seem to compile with clang -O0. > +Note that ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d), where the helper function integer > +would be read from a specified register, is reserved and currently not permitted. > + > Atomic operations > ================= > > diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst > index 2d3fe59bd26..89a13f1cdeb 100644 > --- a/Documentation/bpf/instruction-set.rst > +++ b/Documentation/bpf/instruction-set.rst > @@ -191,7 +191,7 @@ 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 > @@ -202,6 +202,26 @@ 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 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. > + > +Note that ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d), where the helper function integer > +would be read from a specified register, is reserved and currently not permitted. > > Load and store instructions > =========================== > -- > 2.33.4 > > -- > Bpf mailing list > Bpf@xxxxxxxx > https://www.ietf.org/mailman/listinfo/bpf
