• Masahiro Yamada's avatar
    kbuild: do not create *.prelink.o for Clang LTO or IBT · c25e1c55
    Masahiro Yamada authored
    When CONFIG_LTO_CLANG=y, additional intermediate *.prelink.o is created
    for each module. Also, objtool is postponed until LLVM IR 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
    similar 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 IR)          (ELF)       |    (ELF)
                                                                 |
                                                     foo.mod.o --/
                                                     (LLVM IR)
    
     [2] multi-object module
                                          $(LD)
               $(CC)         $(AR)       +objtool               $(LD)
        foo1.c -----> foo1.o -----> foo.o -----> foo.prelink.o -----> foo.ko
                               |  (archive)          (ELF)       |    (ELF)
        foo2.c -----> foo2.o --/                                 |
                     (LLVM IR)                       foo.mod.o --/
                                                     (LLVM IR)
    
      One confusion is that 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, there is no need to keep the LLVM IR.
      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)     |      (ELF)
                                                      |
                                          foo.mod.o --/
                                          (LLVM IR)
    
     [2] multi-object module
    
      Previously, $(AR) was used to combine LLVM IR files into an archive,
      but there was no technical reason to do so. Use $(LD) to merge them
      into a single ELF object.
    
                                   $(LD)
                 $(CC)            +objtool          $(LD)
        foo1.c ---------> foo1.o ---------> foo.o ---------> foo.ko
                                     |      (ELF)     |      (ELF)
        foo2.c ---------> foo2.o ----/                |
                         (LLVM IR)        foo.mod.o --/
                                          (LLVM IR)
    Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
    Reviewed-by: default avatarNicolas Schier <nicolas@fjasle.eu>
    Tested-by: default avatarNathan Chancellor <nathan@kernel.org>
    Reviewed-by: default avatarSami Tolvanen <samitolvanen@google.com>
    Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14 (x86-64)
    Acked-by: default avatarJosh Poimboeuf <jpoimboe@kernel.org>
    c25e1c55
Makefile.modpost 4.2 KB