Date:   Mon, 5 Oct 2020 12:27:39 -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 10:16 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> 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 $@

This is an extremely unusual set of steps, and might be worthwhile to
leave a comment explaining what's going on, so that I or someone else
doesn't ask the same question few months later :)

At any rate, this fixes the issue, so:

Acked-by: Andrii Nakryiko <andrii@...nel.org>

> >
> > 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.

Ok, right, thanks for explanation. I wonder if this "native" clang +
llc pass will also help with vmlinux.h on 32-bit architectures (though
with my recent patches that shouldn't be necessary, so this is more of
a curiosity, rather than the need).

>
> 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.

Right, of course.

>
> > relevant and necessary, or we can just simplify it now?
> >
> >>   ifeq ($(DWARF2BTF),y)
> >>          $(BTF_PAHOLE) -J $@
> >>   endif
> >> --
> >> 2.24.1
> >>

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