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

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Date:   Mon, 19 Mar 2018 20:57:48 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH 1/2] x86: Force asm-goto

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@...radead.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?

Thanks,

	tglx