Re: [PATCH 1/2] x86: Force asm-goto

From: Anders Roxell
Date: Tue Mar 27 2018 - 13:11:56 EST


On 2018-03-19 21:13, Peter Zijlstra wrote:
> 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) <peterz@xxxxxxxxxxxxx>
> > > ---
> > > 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 <peterz@xxxxxxxxxxxxx>
> 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.

With this patch applied I'm not able to do "make kernelversion" without
any toolchain to get the kernel version, I was able to do that before.

Cheers,
Anders

>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
> 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