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From: Yonghong Song <yhs@fb.com>
To: Alan Maguire <alan.maguire@oracle.com>
Cc: <ast@kernel.org>, <daniel@iogearbox.net>, <andrii@kernel.org>,
	<kafai@fb.com>, <songliubraving@fb.com>,
	<john.fastabend@gmail.com>, <kpsingh@chromium.org>,
	<rostedt@goodmis.org>, <mingo@redhat.com>, <haoluo@google.com>,
	<jolsa@kernel.org>, <quentin@isovalent.com>,
	<linux-kernel@vger.kernel.org>, <netdev@vger.kernel.org>,
	<bpf@vger.kernel.org>, <shuah@kernel.org>, <lmb@cloudflare.com>,
	<linux-kselftest@vger.kernel.org>
Subject: Re: [PATCH v2 bpf-next 0/3] bpf: support module BTF in BTF display helpers
Date: Sun, 6 Dec 2020 19:38:26 -0800	[thread overview]
Message-ID: <a17e79e9-5db1-9ff2-6ffc-71249e7ae3e8@fb.com> (raw)
In-Reply-To: <alpine.LRH.2.23.451.2012060038260.1505@localhost>



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
> 

  reply	other threads:[~2020-12-07  3:40 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-04 18:48 Alan Maguire
2020-12-04 18:48 ` [PATCH v2 bpf-next 1/3] bpf: eliminate btf_module_mutex as RCU synchronization can be used Alan Maguire
2020-12-04 18:48 ` [PATCH v2 bpf-next 2/3] bpf: add module support to btf display helpers Alan Maguire
2020-12-04 18:48 ` [PATCH v2 bpf-next 3/3] selftests/bpf: verify module-specific types can be shown via bpf_snprintf_btf Alan Maguire
2020-12-05 20:35 ` [PATCH v2 bpf-next 0/3] bpf: support module BTF in BTF display helpers Yonghong Song
2020-12-05 20:39   ` Yonghong Song
2020-12-06  0:43   ` Alan Maguire
2020-12-07  3:38     ` Yonghong Song [this message]
2020-12-08  3:42     ` Andrii Nakryiko

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