On Mon, Dec 10, 2018 at 3:46 PM Nathan Chancellor <natechancel...@gmail.com> wrote: > > When building an allyesconfig build with Clang, the kernel fails to link > arch/x86/platform/efi/efi_64.o because of a failed BUILD_BUG_ON: > > ld: arch/x86/platform/efi/efi_64.o: in function > `efi_sync_low_kernel_mappings': > (.text+0x8e5): undefined reference to `__compiletime_assert_277' > > Since there are several BUILD_BUG_ONs in efi_64.c, I isolated it down to > the following: > > BUILD_BUG_ON(p4d_index(EFI_VA_END) != p4d_index(MODULES_END)); > > After some research, it turns out that there is a new config option > called NO_AUTO_INLINE, which adds '-fno-inline-functions' to > KBUILD_CFLAGS so that the compiler does not auto inline small functions, > which causes this BUILD_BUG_ON to fail because p4d_index is no longer an > integer constant expression. > > According to the help text of the config, functions explicitly marked > inline should still be inlined. As it turns out, GCC and Clang both > support '-fno-inline-functions' but Clang only inlines functions when > they are marked with an always inline annotation[1]. > > Since it's expected that p4d_index should always be inlined so that its > value can be evaluated at build time, mark it as __always_inline. > > [1]: > https://github.com/llvm-mirror/clang/blob/master/lib/Frontend/CompilerInvocation.cpp#L637-L656
(resending as text-plain; why does gmail silently switch it back to html...) Just some top secret github-fu: this link is bound to break over time as the source changes. While on that page, if you hit `y`, it's hotkeyed to update the URL to pin it to a SHA. That is a little more robust (the branch or even the tree could still disappear, or history gets rewritten (hopefully not on a master branch, but I've seen it happen accidentally (Firefox OS))). This doesn't look like an invasive change to me, though I'm curious about the usefulness of CONFIG_NO_AUTO_INLINE if the kernel is still compiled at -O2. Reviewed-by: Nick Desaulniers <ndesaulni...@google.com> Thanks for sending! > > Link: https://github.com/ClangBuiltLinux/linux/issues/256 > Signed-off-by: Nathan Chancellor <natechancel...@gmail.com> > --- > arch/x86/include/asm/pgtable.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h > index 40616e805292..f78e53382498 100644 > --- a/arch/x86/include/asm/pgtable.h > +++ b/arch/x86/include/asm/pgtable.h > @@ -918,7 +918,7 @@ static inline int p4d_bad(p4d_t p4d) > } > #endif /* CONFIG_PGTABLE_LEVELS > 3 */ > > -static inline unsigned long p4d_index(unsigned long address) > +static __always_inline unsigned long p4d_index(unsigned long address) > { > return (address >> P4D_SHIFT) & (PTRS_PER_P4D - 1); > } > -- > 2.20.0 > -- Thanks, ~Nick Desaulniers
