On Mon, Jun 19, 2023 at 2:17 PM Yonghong Song <yhs@xxxxxxxx> wrote: > > How many people really build the kernel with > LLVM=1 LLVM_IAS=0 > which uses clang compiler ans gcc 'as'. > I think distro most likely won't do this if they intend to > build the kernel with clang. > > Note that > LLVM=1 > implies to use both clang compiler and clang assembler. Yes, we prefer folks to build with LLVM=1. The problem exists for users of stable kernels that predate LLVM_IAS=1 support working well (4.19 is when we had most of the assembler related issues sorted out, actually later but we backported most fixes to 4.19). > > Using clang17 and 'LLVM=1 LLVM_IAS=0', with latest bpf-next, > I actually hit some build errors like: > > /tmp/video-bios-59fa52.s: Assembler messages: > /tmp/video-bios-59fa52.s:4: Error: junk at end of line, first > unrecognized character is `"' Probably because: 1. CONFIG_DEBUG_INFO_DWARF5=y was set or CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y and you're using a version of clang which implicitly defaults to DWARFv5. 2. you're using a version of GAS that does not understand DWARFv5. 3. you did not run defconfig/menuconfig to have kconfig check for DWARFv5 support. The kconfigs should prevent you from selecting DWARFv5 if your toolchain combination doesn't support it; if you run kconfig. > /tmp/video-bios-59fa52.s:4: Error: file number less than one > /tmp/video-bios-59fa52.s:5: Error: junk at end of line, first > unrecognized character is `"' > /tmp/video-bios-59fa52.s:6: Error: junk at end of line, first > unrecognized character is `"' > /tmp/video-bios-59fa52.s:7: Error: junk at end of line, first > unrecognized character is `"' > /tmp/video-bios-59fa52.s:8: Error: junk at end of line, first > unrecognized character is `"' > /tmp/video-bios-59fa52.s:9: Error: junk at end of line, first > unrecognized character is `"' > /tmp/video-bios-59fa52.s:10: Error: junk at end of line, first > unrecognized character is `"' > /tmp/video-bios-59fa52.s:68: Error: junk at end of line, first > unrecognized character is `"' > clang: error: assembler command failed with exit code 1 (use -v to see > invocation) > make[4]: *** [/home/yhs/work/bpf-next/scripts/Makefile.build:252: > arch/x86/realmode/rm/video-bios.o] Error 1 > make[4]: *** Waiting for unfinished jobs.... > /tmp/wakemain-88777c.s: Assembler messages: > /tmp/wakemain-88777c.s:4: Error: junk at end of line, first unrecognized > character is `"' > /tmp/wakemain-88777c.s:4: Error: file number less than one > /tmp/wakemain-88777c.s:5: Error: junk at end of line, first unrecognized > character is `"' > /tmp/wakemain-88777c.s:6: Error: junk at end of line, first unrecognized > character is `"' > /tmp/wakemain-88777c.s:7: Error: junk at end of line, first unrecognized > character is `"' > /tmp/wakemain-88777c.s:8: Error: junk at end of line, first unrecognized > character is `"' > /tmp/wakemain-88777c.s:81: Error: junk at end of line, first > unrecognized character is `"' > /tmp/wakemain-88777c.s:312: Error: junk at end of line, first > unrecognized character is `"' > clang: error: assembler command failed with exit code 1 (use -v to see > invocation) > > Potentially because of my local gnu assembler 2.30-120.el8 won't work It's recorded in lib/Kconfig.debug that 2.35.2 is required for DWARFv5 support if you're using GAS. My machine has 2.40. > with some syntax generated by clang. Mixing clang compiler and arbitrary > gnu assembler are not a good idea (see the above example). It might I agree, but for older branches of stable which are still supported, we didn't quite have clang assembler support usable. We still need to support those branches of stable. > work with close-to-latest gnu assembler. > > To support function name like '<fname>.isra', some llvm work will be > needed, and it may take some time. > > So in my opinion, this patch is NOT a bug fix. It won't affect distro. > Whether we should backport to the old kernel, I am not sure whether it > is absolutely necessary as casual build can always remove LLVM_IAS=0 or > hack the kernel source itself. -- Thanks, ~Nick Desaulniers
