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Date:   Tue, 30 Oct 2018 11:09:54 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Masahiro Yamada <yamada.masahiro@socionext.com>
Cc:     zhenzhong.duan@oracle.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>, konrad.wilk@oracle.com,
        David Woodhouse <dwmw@amazon.co.uk>,
        Thomas Gleixner <tglx@linutronix.de>, srinivas.eeda@oracle.com,
        Borislav Petkov <bp@suse.de>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Michal Marek <michal.lkml@markovi.net>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 1/3] retpolines: Only enable retpoline when compiler
 support it
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On Tue, Oct 30, 2018 at 06:39:24PM +0900, Masahiro Yamada wrote:
> Hi,
> 
> 
> 
> On Tue, Oct 30, 2018 at 3:57 PM Zhenzhong Duan
> <zhenzhong.duan@oracle.com> wrote:
> >
> > Since retpoline capable compilers are widely available, make
> > CONFIG_RETPOLINE hard depend on it.
> >
> > Change KBUILD to use CONFIG_RETPOLINE_SUPPORT to avoid conflict with
> > CONFIG_RETPOLINE which is used by kernel.
> >
> > With all that stuff, the check of RETPOLINE is changed to
> > CONFIG_RETPOLINE.
> >
> > This change is based on suggestion in https://lkml.org/lkml/2018/9/18/1016
> >
> > Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Borislav Petkov <bp@suse.de>
> > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > Cc: David Woodhouse <dwmw@amazon.co.uk>
> > Cc: H. Peter Anvin <hpa@zytor.com>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> > Cc: Michal Marek <michal.lkml@markovi.net>
> > ---
> 
> 
> Instead of adding another CONFIG option,
> does it make sense to add compiler support checks
> to 'depends on' syntax ?
> 
> 
> config RETPOLINE
>          bool "Avoid speculative indirect branches in kernel"
>          depends on $(cc-option,-mindirect-branch=thunk-extern
> -mindirect-branch-register) || \
>                     $(cc-option,-mretpoline-external-thunk)
>          default y
>          select STACK_VALIDATION if HAVE_STACK_VALIDATION

That seems to be what we did for stackprotector, which is similar in
that it used to fail the build. So yes, this seems sane.