[PATCH v2] x86/pgtable: Always inline p4d_index

[Nathan Chancellor <natechancellor@gmail.com>]

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@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
---

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);
 }
-- 
2.20.1