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    SubjectRe: [PATCH] x86/pgtable: Always inline p4d_index
    On Mon, Dec 10, 2018 at 3:46 PM Nathan Chancellor
    <natechancellor@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 <ndesaulniers@google.com>
    Thanks for sending!

    >
    > Link: https://github.com/ClangBuiltLinux/linux/issues/256
    > Signed-off-by: Nathan Chancellor <natechancellor@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

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