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Date: Mon, 7 Dec 2020 19:42:19 -0800 From: Andrii Nakryiko <andrii.nakryiko@...il.com> To: Alan Maguire <alan.maguire@...cle.com> Cc: Yonghong Song <yhs@...com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>, john fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...omium.org>, Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...hat.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Quentin Monnet <quentin@...valent.com>, open list <linux-kernel@...r.kernel.org>, Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, Shuah Khan <shuah@...nel.org>, Lorenz Bauer <lmb@...udflare.com>, "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org> Subject: Re: [PATCH v2 bpf-next 0/3] bpf: support module BTF in BTF display helpers On Sat, Dec 5, 2020 at 4:44 PM Alan Maguire <alan.maguire@...cle.com> 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 Let's instead of the real veth module use selftests's bpf_testmod, which I added specifically to use for tests like these. We can define whatever types we need in there. > 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
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