Re: [PATCH v9 3/3] kbuild: distributed build support for Clang ThinLTO

[Rong Xu]

On Fri, May 22, 2026 at 10:52 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
>
>
> On 5/22/26 8:32 AM, Rong Xu wrote:
> > On Thu, May 21, 2026 at 2:57 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
> >>
> >>
> >> On 5/21/26 11:31 AM, Rong Xu wrote:
> >>> Yonghong, thanks for the update.
> >>>
> >>> Regarding this guard: ther is a period of Clang (before this patch and
> >>> after your first patch), even though ld.lld having these options
> >>> (specifically --lto-whole-program-visibility -mllvm
> >>> -always-rename-promoted-locals=false), distributed ThinLTO mode
> >>> remains unsupported, correct? What the behvior of using this options
> >>> in distributed mode with these compilers? nop or it will lead to
> >>> error?
> >> The in-process thin-lto support is landed on Feb 27.
> >> The distributed thin-lto support is landed on Apr 24.
> >>
> >> If people are using distributed thin-lto in kernel between Feb 27 and
> >> Apr 24, there will be some issues. But people typically use released
> >> compiler, so we should be fine.
> > This is not the case for us (google). We do use compiler b/w releases,
> > and we build our own.
> >
> > What is the issue if we use the compiler in b/w Feb27 and Apr24?
>
> If you use the custom compiler between Feb27 and Apr24 and your kernel
> will do distributed thin-lto, you can remove
>     $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
> from your kernel. Since you have custom compiler, you can
> do some customization for your kernel as well.

I am referring to this specific patch -- there are cases that use
custom compilers built between the February 27 and April 24 LLVM
releases.
I don't want to see any breakage for distributed ThinLTO in these cases.

I would prefer to keep this guard for distributed ThinLTO for the time
being and remove it later. What do you think?


>
>
> >
> > -Rong
> >
> >>> I would assume there will be errors; otherwise, you would not ask me
> >>> to change my patch last time. In this case, I would keep this guard
> >>> and remove it when the minimum llvm version passes llvm23. What do you
> >>> think?
> >> There is no need to keep compiler version guard.
> >>
> >> Before llvm23, the below will be a noop:
> >>      $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
> >> since '-mllvm -always-rename-promoted-locals=false' is a new flag and the compiler won't
> >> recognize it so the kernel will resolve above 'call ...' option as noop.
> >>
> >> With llvm23 and later, the kernel will be able to recognize above options and
> >> things should be okay.
> >>
> >>> Best,
> >>>
> >>> -Rong
> >>>
> >>>
> >>> On Thu, May 21, 2026 at 1:57 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
> >>>>
> >>>> On 3/31/26 9:27 AM, Nathan Chancellor wrote:
> >>>>> Hi Rong,
> >>>>>
> >>>>> On Tue, Mar 31, 2026 at 03:48:27PM +0000, xur@xxxxxxxxxx wrote:
> >>>>>> diff --git a/Makefile b/Makefile
> >>>>>> index 69ccf9b8507d..26005c64016d 100644
> >>>>>> --- a/Makefile
> >>>>>> +++ b/Makefile
> >>>>>> @@ -1047,11 +1047,13 @@ export CC_FLAGS_SCS
> >>>>>>     endif
> >>>>>>
> >>>>>>     ifdef CONFIG_LTO_CLANG
> >>>>>> -ifdef CONFIG_LTO_CLANG_THIN
> >>>>>> +ifdef CONFIG_LTO_CLANG_FULL
> >>>>>> +CC_FLAGS_LTO        := -flto
> >>>>>> +else
> >>>>>>     CC_FLAGS_LTO       := -flto=thin -fsplit-lto-unit
> >>>>>> +if CONFIG_LTO_CLANG_THIN
> >>>>> This should be an 'ifdef', not an 'if'. You copied Yonghong's mistake:
> >>>>>
> >>>>>      https://lore.kernel.org/abgRRX3PH9IaExi8@xxxxxxxxxxxxx/
> >>>>>      https://lore.kernel.org/6db3a2f6-d61c-42f1-9b9d-0aca021cc2d7@xxxxxxxxx/
> >>>>>
> >>>>> Please slow down and test build your changes before sending them. Each
> >>>>> revision adds four new emails to everyone's inbox, which is just noise
> >>>>> when there are obvious, basic problems. 'b4 diff' shows no actual
> >>>>> difference from v8 and v9, which should have been caught by a simple
> >>>>> build test right before 'git send-email'.
> >>>>>
> >>>>>>     KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
> >>>>>> -else
> >>>>>> -CC_FLAGS_LTO        := -flto
> >>>>>> +endif
> >>>> The above patch has a guard CONFIG_LTO_CLANG_THIN, which can be removed.
> >>>> See llvm patch
> >>>>       https://github.com/llvm/llvm-project/pull/188074
> >>>> which supports distributed thin-lto mode too for reducing the number
> >>>> of renaming. In other words, for llvm23, both in-process and
> >>>> distributed-process are supported for thin-lto.
> >>>>
> >>>>>>     endif
> >>>>>>     CC_FLAGS_LTO       += -fvisibility=hidden
> >>>>>>
> >>>>>> @@ -1657,6 +1659,7 @@ endif # CONFIG_MODULES
> >>>>>>     CLEAN_FILES += vmlinux.symvers modules-only.symvers \
> >>>>>>               modules.builtin modules.builtin.modinfo modules.nsdeps \
> >>>>>>               modules.builtin.ranges vmlinux.o.map vmlinux.unstripped \
> >>>>>> +           vmlinux.thinlto-index builtin.order \
> >>>>>>               compile_commands.json rust/test \
> >>>>>>               rust-project.json .vmlinux.objs .vmlinux.export.c \
> >>>>>>                    .builtin-dtbs-list .builtin-dtb.S
> >>>>>> @@ -2118,7 +2121,7 @@ clean: $(clean-dirs)
> >>>>>>        $(call cmd,rmfiles)
> >>>>>>        @find . $(RCS_FIND_IGNORE) \
> >>>>>>                \( -name '*.[aios]' -o -name '*.rsi' -o -name '*.ko' -o -name '.*.cmd' \
> >>>>>> -            -o -name '*.ko.*' \
> >>>>>> +            -o -name '*.ko.*' -o -name '*.o.thinlto.bc' \
> >>>>>>                -o -name '*.dtb' -o -name '*.dtbo' \
> >>>>>>                -o -name '*.dtb.S' -o -name '*.dtbo.S' \
> >>>>>>                -o -name '*.dt.yaml' -o -name 'dtbs-list' \
> >>>>> With that addressed above, the series survives my basic LLVM 22.1.2
> >>>>> build test with my distribution configuration. I'll provide formal tags
> >>>>> on a properly tested and fixed revision.
> >>>>>
> >>>>> Cheers,
> >>>>> Nathan
>