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Mon, 7 Dec 2020 03:38:30 +0000 Subject: Re: [PATCH v2 bpf-next 0/3] bpf: support module BTF in BTF display helpers To: Alan Maguire CC: , , , , , , , , , , , , , , , , , References: <1607107716-14135-1-git-send-email-alan.maguire@oracle.com> <3dce8546-60d4-bb94-2c7a-ed352882cd37@fb.com> From: Yonghong Song Message-ID: Date: Sun, 6 Dec 2020 19:38:26 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [2620:10d:c090:400::5:209a] X-ClientProxiedBy: MW4PR04CA0167.namprd04.prod.outlook.com (2603:10b6:303:85::22) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2620:10d:c085:21c8::113f] (2620:10d:c090:400::5:209a) by MW4PR04CA0167.namprd04.prod.outlook.com (2603:10b6:303:85::22) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3632.21 via Frontend Transport; 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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 >