Re: [Bpf] ISA: BPF_CALL | BPF_X

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> clang generates BPF code with opcode 0x8d (BPF_CALL | BPF_X, which it
> calls "callx"), when compiling with -O0 or -O1.  Of course -O2 is
> recommended, but if anyone later defines opcode 0x8d for anything
> other than what clang means by it, it could cause problems.

GCC also generates BPF_CALL|BPF_X also named callx, but only if the
experimental -mxbpf option is passed to the compiler.

I recommend this particular encoding to be specifically reserved for a
future `call REG' for when/if a time comes when the BPF verifier
supports some form of indirect calls.

>
> On the BPF_MSH thread at
> https://mailarchive.ietf.org/arch/msg/bpf/ogmS9qFhdBCxC4VrOWL7nzjSiXU/
> Alexei wrote regarding BPF_ABS and BPF_IND:
>> DW never existed in classic bpf, so abs/ind never had DW flavor.
>> If some assembler/compiler decided to "support" them it's on them.
>> The standard must not list such things as deprecated. They never existed.
>
> Technically BPF_CALL | BPF_X never existed either, so can be omitted from
> the IANA registry.  But given the widespread deployment of clang's
> use, and the WG charter statement:
>> The BPF working group is initially tasked with documenting the existing
>> state of the BPF ecosystem
> I could see a potential argument to list it as reserved or something.
>
> Today, the document doesn't reserve it, so it's open for future use for any
> purpose.  I just wanted to verify that the WG is ok with not listing it
> in the IANA registry, given the above information.
>
> Dave