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SubjectRe: [PATCH 1/2] x86: Force asm-goto
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@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 <peterz@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) <peterz@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
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