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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 22 Jul 2019 10:21:07 -0700
Message-ID: <CAKwvOd=KRVsFkT8dLFoitky9OF8tKmbn00-OPi6kBygyx4QwHg@mail.gmail.com>
Subject: Re: [PATCH v2] powerpc: slightly improve cache helpers
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Nathan Chancellor <natechancellor@gmail.com>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev@lists.ozlabs.org, LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Sun, Jul 21, 2019 at 11:19 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Sun, Jul 21, 2019 at 07:41:40PM -0700, Nathan Chancellor wrote:
> > Hi Segher,
> >
> > On Sun, Jul 21, 2019 at 01:01:50PM -0500, Segher Boessenkool wrote:
> > > On Sun, Jul 21, 2019 at 12:58:46AM -0700, Nathan Chancellor wrote:
> > > > 0000017c clear_user_page:
> > > >      17c: 94 21 ff f0                     stwu 1, -16(1)
> > > >      180: 38 80 00 80                     li 4, 128
> > > >      184: 38 63 ff e0                     addi 3, 3, -32
> > > >      188: 7c 89 03 a6                     mtctr 4
> > > >      18c: 38 81 00 0f                     addi 4, 1, 15
> > > >      190: 8c c3 00 20                     lbzu 6, 32(3)
> > > >      194: 98 c1 00 0f                     stb 6, 15(1)
> > > >      198: 7c 00 27 ec                     dcbz 0, 4
> > > >      19c: 42 00 ff f4                     bdnz .+65524
> > >
> > > Uh, yeah, well, I have no idea what clang tried here, but that won't
> > > work.  It's copying a byte from each target cache line to the stack,
> > > and then does clears the cache line containing that byte on the stack.
> > >
> > > I *guess* this is about "Z" and not about "%y", but you'll have to ask
> > > the clang people.
> > >
> > > Or it may be that they do not treat inline asm operands as lvalues
> > > properly?  That rings some bells.  Yeah that looks like it.
>
> The code is
>   __asm__ __volatile__ ("dcbz %y0" : : "Z"(*(u8 *)addr) : "memory");
>
> so yeah it looks like clang took that  *(u8 *)addr  as rvalue, and
> stored that in stack, and then used *that* as memory.

What's the %y modifier supposed to mean here?  addr is in the list of
inputs, so what's wrong with using it as an rvalue?

>
> Maybe clang simply does not not to treat "Z" the same as "m"?  (And "Y"
> and "Q" and "es" and a whole bunch of "w*", what about those?)

-- 
Thanks,
~Nick Desaulniers