Re: [PATCH 0/9] eBPF support for GNU binutils

[jose.marchesi@oracle.com (Jose E. Marchesi)]

Hi Alexey.

    > > Despite using a different syntax for the assembler (the llvm assembler
    > > uses a C-ish expression-based syntax while the GNU assembler opts for
    > > a more classic assembly-language syntax) this implementation tries to
    > > provide inter-operability with clang/llvm generated objects.
    >
    > I also noticed your implementation doesn’t seem to use the same sub-register
    > syntax as what LLVM assembler is doing.
    >
    >   x register for 64-bit, and w register for 32-bit sub-register.
    >
    > So:
    >   add r0, r1, r2 means BPF_ALU64 | BPF_ADD | BFF_X
    >   add w0, w1, w1 means BPF_ALU | BPF_ADD | BPF_X
    >
    > ASAICT, different register prefix for different register width is also adopted
    > by quite a few other GNU assembler targets like AArch64, X86_64.
    
    there is also Ed's assembler:
    https://github.com/solarflarecom/ebpf_asm

Oh interesting, didn't know about it.

    It uses 2 ops style.
    I think 3 ops style "add r0,r1,r2" is not a good fit for bpf isa.

I agree.  My port uses two-operand formats for most instructions,
including arithmetic instructions.  Examples:

sub %r2, 666
sub %r3, -666
sub %r4, 0x7eadbeef
sub %r5, %r6
    
    I think we need to converge on one asm syntax for gas/bfd.
    At this point we cannot change llvm's asm output,
    so my preference would be to make gas accept it.
    But I understand the implementation difficulties to fit it into bfd infra.
    So I'm ok with more traditional asm the way Dave implemented it few
    years back.
    One asm syntax for gas and another asm syntax for clang is, imo, acceptable.
    
    Jose, can you combine Dave's patches with yours?

As far as I can see my port uses basically the same syntax than Dave's.

The only significant difference is that mine uses a % prefix for
register names.  I prefer it this way in order to avoid collisions with
symbols.
    
    I think Ed had an idea on how to specify BTF in asm syntax.
    BTF has to be supported by the assembler as well
    along with .btf.ext, lineinfo, etc
    Currently llvm emits btf as '.byte 0x...', but that's far from ideal.

I haven't looked into BTF yet, but it would be certainly good, for
everyone to use the same directives to support it :)