linux-kernel - Re: [PATCH] arch/arm64/include/asm/jump_label.h: use asm_volatile

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Date:   Mon, 10 Sep 2018 10:02:09 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch/arm64/include/asm/jump_label.h: use asm_volatile_goto

On Mon, Sep 10, 2018 at 2:49 AM Will Deacon <will.deacon@....com> wrote:
>
> On Sun, Sep 09, 2018 at 05:47:31PM +0200, Miguel Ojeda wrote:
> > All other uses of "asm goto" go through asm_volatile_goto
> > (including the arm version of the same file). For consistency,
> > use it here as well.
> >
> > Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> > Cc: Catalin Marinas <catalin.marinas@....com>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: linux-arm-kernel@...ts.infradead.org
> > Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
> > ---
> >  arch/arm64/include/asm/jump_label.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
>
> Thanks, I'll pick this up as a fix.
>
> Will
>
> > diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
> > index 1b5e0e843c3a..7e2b3e360086 100644
> > --- a/arch/arm64/include/asm/jump_label.h
> > +++ b/arch/arm64/include/asm/jump_label.h
> > @@ -28,7 +28,7 @@
> >
> >  static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
> >  {
> > -     asm goto("1: nop\n\t"
> > +     asm_volatile_goto("1: nop\n\t"
> >                ".pushsection __jump_table,  \"aw\"\n\t"
> >                ".align 3\n\t"
> >                ".quad 1b, %l[l_yes], %c0\n\t"
> > @@ -42,7 +42,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
> >
> >  static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
> >  {
> > -     asm goto("1: b %l[l_yes]\n\t"
> > +     asm_volatile_goto("1: b %l[l_yes]\n\t"
> >                ".pushsection __jump_table,  \"aw\"\n\t"
> >                ".align 3\n\t"
> >                ".quad 1b, %l[l_yes], %c0\n\t"
> > --
> > 2.17.1
> >

Yes, this should help avoid a known miscompile in gcc < 4.8.2 (at
least for x86_64; not sure how backend specific the asm goto
implementation is in gcc, but this shouldn't hurt).
https://lkml.org/lkml/2018/9/7/1628

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

-- 
Thanks,
~Nick Desaulniers