From: Sami Tolvanen <samitolvanen@google.com>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild <linux-kbuild@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Nicolas Schier <nicolas@fjasle.eu>,
Peter Zijlstra <peterz@infradead.org>,
linux-modules@vger.kernel.org, linux-s390@vger.kernel.org,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
clang-built-linux <clang-built-linux@googlegroups.com>,
Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH v4 09/14] kbuild: do not create *.prelink.o for Clang LTO or IBT
Date: Mon, 9 May 2022 16:13:11 -0700 [thread overview]
Message-ID: <CABCJKudnbTS=_2WgX63xb_3oCwt_6jwotqfoV5wSjkmfjuUJZQ@mail.gmail.com> (raw)
In-Reply-To: <20220508190631.2386038-10-masahiroy@kernel.org>
On Sun, May 8, 2022 at 12:10 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> When CONFIG_LTO_CLANG=y, additional intermediate *.prelink.o is created
> for each module. Also, objtool is postponed until LLVM bitcode is
> converted to ELF.
>
> CONFIG_X86_KERNEL_IBT works in a similar way to postpone objtool until
> objects are merged together.
>
> This commit stops generating *.prelink.o, so the build flow will look
> the same with/without LTO.
>
> The following figures show how the LTO build currently works, and
> how this commit is changing it.
>
> Current build flow
> ==================
>
> [1] single-object module
>
> $(LD)
> $(CC) +objtool $(LD)
> foo.c --------------------> foo.o -----> foo.prelink.o -----> foo.ko
> (LLVM bitcode) (ELF) |
> |
> foo.mod.o --/
>
> [2] multi-object module
> $(LD)
> $(CC) $(AR) +objtool $(LD)
> foo1.c -----> foo1.o -----> foo.o -----> foo.prelink.o -----> foo.ko
> | (archive) (ELF) |
> foo2.c -----> foo2.o --/ |
> (LLVM bitcode) foo.mod.o --/
>
> One confusion is foo.o in multi-object module is an archive despite of
> its suffix.
>
> New build flow
> ==============
>
> [1] single-object module
>
> Since there is only one object, we do not need to have the LLVM
> bitcode stage. Use $(CC)+$(LD) to generate an ELF object in one
> build rule. When LTO is disabled, $(LD) is unneeded because $(CC)
> produces an ELF object.
>
> $(CC)+$(LD)+objtool $(LD)
> foo.c ------------------------> foo.o -------> foo.ko
> (ELF) |
> |
> foo.mod.o --/
>
> [2] multi-object module
>
> Previously, $(AR) was used to combine LLVM bitcode into an archive,
> but there was no technical reason to do so.
> This commit just uses $(LD) to combine and convert them into a single
> ELF object.
>
> $(LD)
> $(CC) +objtool $(LD)
> foo1.c -------> foo1.o -------> foo.o -------> foo.ko
> | (ELF) |
> foo2.c -------> foo2.o ---/ |
> (LLVM bitcode) foo.mod.o --/
>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
> Tested-by: Nathan Chancellor <nathan@kernel.org>
Looks good, thanks for cleaning this up!
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Sami
next prev parent reply other threads:[~2022-05-09 23:13 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-08 19:06 [PATCH v4 00/14] kbuild: yet another series of cleanups (modpost, LTO, MODULE_REL_CRCS, export.h) Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 01/14] modpost: remove left-over cross_compile declaration Masahiro Yamada
2022-05-09 17:08 ` Nick Desaulniers
2022-05-12 4:35 ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 02/14] modpost: change the license of EXPORT_SYMBOL to bool type Masahiro Yamada
2022-05-12 4:36 ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 03/14] modpost: split the section mismatch checks into section-check.c Masahiro Yamada
2022-05-09 17:19 ` Nick Desaulniers
2022-05-10 6:55 ` Masahiro Yamada
2022-05-11 18:47 ` Nick Desaulniers
2022-05-11 19:27 ` Masahiro Yamada
2022-05-11 19:51 ` Jeff Johnson
2022-05-11 20:01 ` Nick Desaulniers
2022-05-08 19:06 ` [PATCH v4 04/14] modpost: add sym_find_with_module() helper Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 05/14] modpost: extract symbol versions from *.cmd files Masahiro Yamada
2022-05-09 21:52 ` Sami Tolvanen
2022-05-08 19:06 ` [PATCH v4 06/14] kbuild: link symbol CRCs at final link, removing CONFIG_MODULE_REL_CRCS Masahiro Yamada
2022-05-09 17:50 ` Nick Desaulniers
2022-05-10 13:03 ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 07/14] kbuild: stop merging *.symversions Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 08/14] genksyms: adjust the output format to modpost Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 09/14] kbuild: do not create *.prelink.o for Clang LTO or IBT Masahiro Yamada
2022-05-09 23:13 ` Sami Tolvanen [this message]
2022-05-08 19:06 ` [PATCH v4 10/14] kbuild: check static EXPORT_SYMBOL* by script instead of modpost Masahiro Yamada
2022-05-09 18:05 ` Nick Desaulniers
2022-05-10 13:53 ` Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 11/14] kbuild: make built-in.a rule robust against too long argument error Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 12/14] kbuild: make *.mod " Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 13/14] kbuild: add cmd_and_savecmd macro Masahiro Yamada
2022-05-08 19:06 ` [PATCH v4 14/14] kbuild: rebuild multi-object modules when objtool is updated Masahiro Yamada
2022-05-09 4:24 ` [PATCH v4 00/14] kbuild: yet another series of cleanups (modpost, LTO, MODULE_REL_CRCS, export.h) Masahiro Yamada
2022-05-09 22:13 ` Nathan Chancellor
2022-05-10 6:55 ` Masahiro Yamada
2022-05-09 22:19 ` Nathan Chancellor
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