netdev - Re: [PATCH bpf-next 1/2] samples/bpf: change Makefile to cope with latest llvm

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Date:   Fri, 2 Oct 2020 21:22:11 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...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 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://bugs.llvm.org/ 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://reviews.llvm.org/D87153
> 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
relevant and necessary, or we can just simplify it now?

>  ifeq ($(DWARF2BTF),y)
>         $(BTF_PAHOLE) -J $@
>  endif
> --
> 2.24.1
>