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Date:   Mon, 19 Mar 2018 21:13:27 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     mingo@kernel.org, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org
Subject: Re: [PATCH 1/2] x86: Force asm-goto
Message-ID: <20180319201327.GJ4043@hirez.programming.kicks-ass.net>
References: <20180319154124.337947038@infradead.org>
 <20180319154717.652679334@infradead.org>
 <alpine.DEB.2.21.1803192055410.1520@nanos.tec.linutronix.de>
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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