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

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Date:   Fri, 9 Aug 2019 22:12:56 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     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>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] powerpc: fix inline asm constraints for dcbz

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();                                           \
 }