RE: [Bpf] [PATCH bpf-next v2] bpf, docs: Add callx instructions in new conformance group

[dthaler1968@googlemail.com]

Jose E. Marchesi <jose.marchesi@oracle.com> writes:
> > +BPF_CALL  0x8    0x1  call PC += reg_val(imm)          BPF_JMP | BPF_X
only,
> see `Program-local functions`_
> 
> If the instruction requires a register operand, why not using one of the
> register fields?  Is there any reason for not doing that?

Yeah, the reason is because this is document what clang has done by default
for a long time now.  The IETF WG charter says:

> The BPF working group is initially tasked with documenting the existing
> state of the BPF ecosystem

So extensions can always add new instructions and deprecate old ones
but the initial version of the ISA needs to document "the existing state
of the BPF ecosystem".  I know gcc used a different field but one has to
go out of your way to specify a command line option to get that to happen,
whereas clang uses callx as documented when you don't do -O2, without
requiring any extra command line options.

I agree with you that it would have been better to use the src register
since the BPF_X bit is supposed to mean that, but that ship apparently
sailed long ago with clang.

Dave