linux-kernel - Re: [PATCH] powerpc: fix inline asm constraints for dcbz

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Date:   Fri, 9 Aug 2019 15:03:30 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        kbuild test robot <lkp@...el.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH] powerpc: fix inline asm constraints for dcbz

On Fri, Aug 9, 2019 at 1:13 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Fri, Aug 9, 2019 at 10:02 PM Christophe Leroy
> <christophe.leroy@....fr> wrote:
> >
> > Arnd Bergmann <arnd@...db.de> a écrit :
> > > On Fri, Aug 9, 2019 at 8:21 PM 'Nick Desaulniers' via Clang Built
> > > Linux <clang-built-linux@...glegroups.com> wrote:
> > >
> > >>  static inline void dcbz(void *addr)
> > >>  {
> > >> -       __asm__ __volatile__ ("dcbz %y0" : : "Z"(*(u8 *)addr) : "memory");
> > >> +       __asm__ __volatile__ ("dcbz %y0" : "=Z"(*(u8 *)addr) :: "memory");
> > >>  }
> > >>
> > >>  static inline void dcbi(void *addr)
> > >>  {
> > >> -       __asm__ __volatile__ ("dcbi %y0" : : "Z"(*(u8 *)addr) : "memory");
> > >> +       __asm__ __volatile__ ("dcbi %y0" : "=Z"(*(u8 *)addr) :: "memory");
> > >>  }
> > >
> > > I think the result of the discussion was that an output argument only kind-of
> > > makes sense for dcbz, but for the others it's really an input, and clang is
> > > wrong in the way it handles the "Z" constraint by making a copy, which it
> > > doesn't do for "m".
> > >
> > > I'm not sure whether it's correct to use "m" instead of "Z" here, which
> > > would be a better workaround if that works. More importantly though,
> > > clang really needs to be fixed to handle "Z" correctly.
> >
> > As the benefit is null, I think the best is probably to reverse my
> > original commit until at least CLang is fixed, as initialy suggested
> > by mpe
>
> Yes, makes sense.
>
> There is one other use of the "Z" constraint, so on top of the revert, I
> think it might be helpful if Nick could check if the patch below makes
> any difference with clang and, if it does, whether the current version
> is broken.
>
>        Arnd
>
> diff --git a/arch/powerpc/include/asm/io.h b/arch/powerpc/include/asm/io.h
> index 23e5d5d16c7e..28b467779328 100644
> --- a/arch/powerpc/include/asm/io.h
> +++ b/arch/powerpc/include/asm/io.h
> @@ -106,7 +106,7 @@ static inline u##size name(const volatile u##size
> __iomem *addr)    \
>  {                                                                      \
>         u##size ret;                                                    \
>         __asm__ __volatile__("sync;"#insn" %0,%y1;twi 0,%0,0;isync"     \
> -               : "=r" (ret) : "Z" (*addr) : "memory");                 \
> +               : "=r" (ret) : "m" (*addr) : "memory");                 \
>         return ret;                                                     \
>  }
>
> @@ -114,7 +114,7 @@ static inline u##size name(const volatile u##size
> __iomem *addr)    \
>  static inline void name(volatile u##size __iomem *addr, u##size val)   \
>  {                                                                      \
>         __asm__ __volatile__("sync;"#insn" %1,%y0"                      \
> -               : "=Z" (*addr) : "r" (val) : "memory");                 \
> +               : "=m" (*addr) : "r" (val) : "memory");                 \
>         mmiowb_set_pending();                                           \
>  }

Does not work:
https://travis-ci.com/ClangBuiltLinux/continuous-integration/builds/122654899
https://github.com/ClangBuiltLinux/continuous-integration/pull/197/files#diff-40bd16e3188587e4d648c30e0c2d6d37

-- 
Thanks,
~Nick Desaulniers