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

[Yonghong Song <yhs@fb.com>]

On 12/5/20 4:43 PM, Alan Maguire wrote:
> 
> 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?

This should work too and we do not need to add a note in
README.rst for this test then.

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