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Date: Sun, 6 Dec 2020 19:38:26 -0800 From: Yonghong Song <yhs@...com> To: Alan Maguire <alan.maguire@...cle.com> CC: <ast@...nel.org>, <daniel@...earbox.net>, <andrii@...nel.org>, <kafai@...com>, <songliubraving@...com>, <john.fastabend@...il.com>, <kpsingh@...omium.org>, <rostedt@...dmis.org>, <mingo@...hat.com>, <haoluo@...gle.com>, <jolsa@...nel.org>, <quentin@...valent.com>, <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>, <bpf@...r.kernel.org>, <shuah@...nel.org>, <lmb@...udflare.com>, <linux-kselftest@...r.kernel.org> Subject: Re: [PATCH v2 bpf-next 0/3] bpf: support module BTF in BTF display helpers 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 >
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