On Mon, Mar 19, 2018 at 08:57:48PM +0100, Thomas Gleixner wrote: > On Mon, 19 Mar 2018, Peter Zijlstra wrote: > > > We want to start using asm-goto to guarantee the absence of dynamic > > branches (and thus speculation). > > > > A primary prerequisite for this is of course that the compiler > > supports asm-goto. This effecively lifts the minimum GCC version to > > build an x86 kernel to gcc-4.5. > > > > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > > --- > > Makefile | 13 +++++++------ > > arch/x86/Makefile | 4 ++++ > > 2 files changed, 11 insertions(+), 6 deletions(-) > > > > --- a/Makefile > > +++ b/Makefile > > @@ -494,6 +494,13 @@ RETPOLINE_CFLAGS_CLANG := -mretpoline-ex > > RETPOLINE_CFLAGS := $(call cc-option,$(RETPOLINE_CFLAGS_GCC),$(call > > cc-option,$(RETPOLINE_CFLAGS_CLANG))) > > export RETPOLINE_CFLAGS > > > > +# check for 'asm goto' > > +ifeq ($(shell $(srctree)/scripts/gcc-goto.sh $(CC) $(KBUILD_CFLAGS)), y) > > Why are you not using the shell-cached version which you remove further > down?
Bah, I'm an idiot. I fixed that up at least two times, but clearly I forgot to check that again before sending. --- Subject: x86: Force asm-goto From: Peter Zijlstra <pet...@infradead.org> Date: Wed Jan 17 11:42:07 CET 2018 We want to start using asm-goto to guarantee the absence of dynamic branches (and thus speculation). A primary prerequisite for this is of course that the compiler supports asm-goto. This effecively lifts the minimum GCC version to build an x86 kernel to gcc-4.5. Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> --- Makefile | 13 +++++++------ arch/x86/Makefile | 4 ++++ 2 files changed, 11 insertions(+), 6 deletions(-) --- a/Makefile +++ b/Makefile @@ -494,6 +494,13 @@ RETPOLINE_CFLAGS_CLANG := -mretpoline-ex RETPOLINE_CFLAGS := $(call cc-option,$(RETPOLINE_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_CFLAGS_CLANG))) export RETPOLINE_CFLAGS +# check for 'asm goto' +ifeq ($(call shell-cached,$(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC) $(KBUILD_CFLAGS)), y) + CC_HAVE_ASM_GOTO := 1 + KBUILD_CFLAGS += -DCC_HAVE_ASM_GOTO + KBUILD_AFLAGS += -DCC_HAVE_ASM_GOTO +endif + ifeq ($(config-targets),1) # =========================================================================== # *config targets only - make sure prerequisites are updated, and descend @@ -658,12 +665,6 @@ KBUILD_CFLAGS += $(call cc-ifversion, -l # Tell gcc to never replace conditional load with a non-conditional one KBUILD_CFLAGS += $(call cc-option,--param=allow-store-data-races=0) -# check for 'asm goto' -ifeq ($(call shell-cached,$(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC) $(KBUILD_CFLAGS)), y) - KBUILD_CFLAGS += -DCC_HAVE_ASM_GOTO - KBUILD_AFLAGS += -DCC_HAVE_ASM_GOTO -endif - include scripts/Makefile.kcov include scripts/Makefile.gcc-plugins --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -181,6 +181,10 @@ ifdef CONFIG_FUNCTION_GRAPH_TRACER endif endif +ifndef CC_HAVE_ASM_GOTO + $(error Compiler lacks asm-goto support.) +endif + # # Jump labels need '-maccumulate-outgoing-args' for gcc < 4.5.2 to prevent a # GCC bug (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46226). There's no way
