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 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
>>