Re: [PATCH v2 bpf-next 0/3] bpf: support module BTF in BTF display helpers

[Alan Maguire]

On Sat, 5 Dec 2020, Yonghong Song wrote:

> 
> 
> __builtin_btf_type_id() is really only supported in llvm12
> and 64bit return value support is pushed to llvm12 trunk
> a while back. The builtin is introduced in llvm11 but has a
> corner bug, so llvm12 is recommended. So if people use the builtin,
> you can assume 64bit return value. libbpf support is required
> here. So in my opinion, there is no need to do feature detection.
> 
> Andrii has a patch to support 64bit return value for
> __builtin_btf_type_id() and I assume that one should
> be landed before or together with your patch.
> 
> Just for your info. The following is an example you could
> use to determine whether __builtin_btf_type_id()
> supports btf object id at llvm level.
> 
> -bash-4.4$ cat t.c
> int test(int arg) {
>   return __builtin_btf_type_id(arg, 1);
> }
> 
> Compile to generate assembly code with latest llvm12 trunk:
>   clang -target bpf -O2 -S -g -mcpu=v3 t.c
> In the asm code, you should see one line with
>   r0 = 1 ll
> 
> Or you can generate obj code:
>   clang -target bpf -O2 -c -g -mcpu=v3 t.c
> and then you disassemble the obj file
>   llvm-objdump -d --no-show-raw-insn --no-leading-addr t.o
> You should see below in the output
>   r0 = 1 ll
> 
> Use earlier version of llvm12 trunk, the builtin has
> 32bit return value, you will see
>   r0 = 1
> which is a 32bit imm to r0, while "r0 = 1 ll" is
> 64bit imm to r0.
>

Thanks for this Yonghong!  I'm thinking the way I'll tackle it
is to simply verify that the upper 32 bits specifying the
veth module object id are non-zero; if they are zero, we'll skip
the test (I think a skip probably makes sense as not everyone will
have llvm12). Does that seem reasonable?

With the additional few minor changes on top of Andrii's patch,
the use of __builtin_btf_type_id() worked perfectly. Thanks!

Alan