[PATCH v5 0/3] extern inline native_save_fl for paravirt

From: Nick Desaulniers
Date: Wed Jun 13 2018 - 17:05:40 EST

paravirt depends on a custom calling convention (callee saved), but
expects this from a static inline function that it then forces to be
outlined. This is problematic because different compilers or flags can
then add a stack guard that violates the calling conventions.

Uses extern inline with the out-of-line definition in assembly to
prevent compilers from adding stack guards to the outlined version.

Other parts of the codebase overwrite KBUILD_CFLAGS, which is *extremely
problematic* for extern inline, as the sematics are completely the
opposite depending on what C standard is used.

Changes since v4:
Take Arnd's and hpa's suggestions properly feature detect gnu_inline
attribute to support gcc 4.1.

Changes since v3:
Take Joe's suggestion to hoist __inline__ and __inline out of
conditional block.

Changes since v2:
Take hpa's _ASM_ARG patch into the set in order to simplify cross
32b/64b x86 assembly and rebase my final patch to use it. Apply
Sedat's typo fix to commit message and sign off on it. Take Joe's
suggestion to simplify __inline__ and __inline. Add Arnd to list of
folks who made helpful suggestions.

Changes since v1:
Prefer gnu_inline function attribute instead of explicitly setting C
standard compiler flag in problematic Makefiles. We should instead
carefully evaluate if those Makefiles should be overwriting
KBUILD_CFLAGS at all. Dropped the previous first two patches and added
a new first patch.

H. Peter Anvin (1):
x86/asm: add _ASM_ARG* constants for argument registers to <asm/asm.h>

Nick Desaulniers (2):
compiler-gcc.h: add gnu_inline to all inline declarations
x86: paravirt: make native_save_fl extern inline

arch/x86/include/asm/asm.h | 59 +++++++++++++++++++++++++++++++++
arch/x86/include/asm/irqflags.h | 2 +-
arch/x86/kernel/Makefile | 1 +
arch/x86/kernel/irqflags.S | 26 +++++++++++++++
include/linux/compiler-gcc.h | 29 ++++++++++++----
5 files changed, 109 insertions(+), 8 deletions(-)
create mode 100644 arch/x86/kernel/irqflags.S