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Date: Fri, 2 Oct 2020 22:15:48 -0700 From: Yonghong Song <yhs@...com> To: Andrii Nakryiko <andrii.nakryiko@...il.com> CC: bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Kernel Team <kernel-team@...com> Subject: Re: [PATCH bpf-next 1/2] samples/bpf: change Makefile to cope with latest llvm On 10/2/20 9:22 PM, Andrii Nakryiko wrote: > On Fri, Oct 2, 2020 at 7:19 PM Yonghong Song <yhs@...com> wrote: >> >> With latest llvm trunk, bpf programs under samples/bpf >> directory, if using CORE, may experience the following >> errors: >> >> LLVM ERROR: Cannot select: intrinsic %llvm.preserve.struct.access.index >> PLEASE submit a bug report to https://urldefense.proofpoint.com/v2/url?u=https-3A__bugs.llvm.org_&d=DwIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=_D9tvuWQ6EbYqcMBdVB0qqRMVdV6Etws5ITtx8Pa1ZM&s=BwTAvhipPl-Az_WaiJDbqU8yl__NvG8W4HmCqWqHdqg&e= and include the crash backtrace. >> Stack dump: >> 0. Program arguments: llc -march=bpf -filetype=obj -o samples/bpf/test_probe_write_user_kern.o >> 1. Running pass 'Function Pass Manager' on module '<stdin>'. >> 2. Running pass 'BPF DAG->DAG Pattern Instruction Selection' on function '@..._prog1' >> #0 0x000000000183c26c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) >> (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x183c26c) >> ... >> #7 0x00000000017c375e (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x17c375e) >> #8 0x00000000016a75c5 llvm::SelectionDAGISel::CannotYetSelect(llvm::SDNode*) >> (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x16a75c5) >> #9 0x00000000016ab4f8 llvm::SelectionDAGISel::SelectCodeCommon(llvm::SDNode*, unsigned char const*, >> unsigned int) (/data/users/yhs/work/llvm-project/llvm/build.cur/install/bin/llc+0x16ab4f8) >> ... >> Aborted (core dumped) | llc -march=bpf -filetype=obj -o samples/bpf/test_probe_write_user_kern.o >> >> The reason is due to llvm change https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D87153&d=DwIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=_D9tvuWQ6EbYqcMBdVB0qqRMVdV6Etws5ITtx8Pa1ZM&s=fo_LvXqHJx_m0m0pJJiDdOUcOzVXm2_iYoxPhpqpzng&e= >> where the CORE relocation global generation is moved from the beginning >> of target dependent optimization (llc) to the beginning >> of target independent optimization (opt). >> >> Since samples/bpf programs did not use vmlinux.h and its clang compilation >> uses native architecture, we need to adjust arch triple at opt level >> to do CORE relocation global generation properly. Otherwise, the above >> error will appear. >> >> This patch fixed the issue by introduce opt and llvm-dis to compilation chain, >> which will do proper CORE relocation global generation as well as O2 level >> optimization. Tested with llvm10, llvm11 and trunk/llvm12. >> >> Signed-off-by: Yonghong Song <yhs@...com> >> --- >> samples/bpf/Makefile | 6 +++++- >> 1 file changed, 5 insertions(+), 1 deletion(-) >> >> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile >> index 4f1ed0e3cf9f..79c5fdea63d2 100644 >> --- a/samples/bpf/Makefile >> +++ b/samples/bpf/Makefile >> @@ -211,6 +211,8 @@ TPROGLDLIBS_xsk_fwd += -pthread >> # make M=samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang >> LLC ?= llc >> CLANG ?= clang >> +OPT ?= opt >> +LLVM_DIS ?= llvm-dis >> LLVM_OBJCOPY ?= llvm-objcopy >> BTF_PAHOLE ?= pahole >> >> @@ -314,7 +316,9 @@ $(obj)/%.o: $(src)/%.c >> -Wno-address-of-packed-member -Wno-tautological-compare \ >> -Wno-unknown-warning-option $(CLANG_ARCH_ARGS) \ >> -I$(srctree)/samples/bpf/ -include asm_goto_workaround.h \ >> - -O2 -emit-llvm -c $< -o -| $(LLC) -march=bpf $(LLC_FLAGS) -filetype=obj -o $@ >> + -O2 -emit-llvm -Xclang -disable-llvm-passes -c $< -o - | \ >> + $(OPT) -O2 -mtriple=bpf-pc-linux | $(LLVM_DIS) | \ >> + $(LLC) -march=bpf $(LLC_FLAGS) -filetype=obj -o $@ > > I keep forgetting exact details of why we do this native clang + llc > pipeline instead of just doing `clang -target bpf`? Is it still samples/bpf programs did not use vmlinux.h. they directly use kernel-devel headers, hence they need to first compile with native arch for clang but later change target arch to bpf to generate final byte code. They cannot just do 'clang -target bpf' without vmlinux.h. But changing to use vmlinux.h is a much bigger project and I merely want to make it just work so people won't make/compile samples/bpf and get compilation errors. > relevant and necessary, or we can just simplify it now? > >> ifeq ($(DWARF2BTF),y) >> $(BTF_PAHOLE) -J $@ >> endif >> -- >> 2.24.1 >>
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