linux-kernel - Re: [PATCH v2] x86/pgtable: Always inline p4d

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Date:   Wed, 16 Jan 2019 09:12:12 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2] x86/pgtable: Always inline p4d_index

On 1/16/19 5:41 AM, Nathan Chancellor 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/llvm-project/blob/84cecfcb3db7/clang/lib/Frontend/CompilerInvocation.cpp#L638-L657
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/256
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Hi,
I see the same build error when using gcc 4.8.5 (SUSE Linux).
This patch fixes that build error.

Acked-by: Randy Dunlap <rdunlap@...radead.org> # build-tested

Thanks.

> ---
> 
> v1 -> v2:
> 
> * Add Nick's Reviewed-by tag.
> 
> * Update the GitHub URL to use the official LLVM repository and pin to a
>   SHA so that it is always valid, as suggested by Nick.
> 
>  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);
>  }
> 


-- 
~Randy