Message-ID: <5ec75c39-2878-4caa-8fbd-68cf0aee28de@linux.dev>
Date: Sun, 26 Jan 2025 19:48:06 -0800
From: Yonghong Song <yonghong.song@...ux.dev>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>, Song Liu <song@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>
Subject: clang thin-lto not working for aarch64 for v6.13

Hi, Masahiro,

We are trying 6.13 kernel and found that for aarch64 thinlto not 
working. For example, for kernel/bpf/syscall.o, the compilation flags 
from .syscall.o.cmd are savedcmd_kernel/bpf/syscall.o := clang 
-Wp,-MMD,kernel/bpf/.syscall.o.d ... -D__KBUILD_MODNAME=kmod_syscall -c 
-o kernel/bpf/syscall.o kernel/bpf/syscall.c ; ld.lld -EL -maarch64elf 
-z norelro -mllvm -import-instr-limit=5 -z noexecstack -r -o 
kernel/bpf/.tmp_syscall.o kernel/bpf/syscall.o; mv 
kernel/bpf/.tmp_syscall.o kernel/bpf/syscall.o I did some bisecting and 
found the issue is due to ``` commit 
bede169618c68379e1be7ace14e8ac85b964a9ec Author: Masahiro Yamada 
<masahiroy@...nel.org> Date: Thu Nov 14 08:45:22 2024 +0900 kbuild: 
enable objtool for *.mod.o and additional kernel objects ``` In the 
above, for thinlto, we should not do ld.lld as compiler needs IR to do 
cross-file inlining. Searching the internet, I found that the issue has 
been reported e.g. in 
https://patchwork.kernel.org/project/linux-kbuild/patch/20241113234526.402738-3-masahiroy@kernel.org/ 
and you mentioned you will fix it. Do you have a fix somewhere? With 
this fix, deploying 6.13 in our production will cause performance 
regression and that is not what we want. Thanks! Yonghong

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