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Date:   Tue, 30 Jul 2019 14:35:02 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Nathan Chancellor <natechancellor@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        christophe leroy <christophe.leroy@c-s.fr>,
        kbuild test robot <lkp@intel.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] powerpc: workaround clang codegen bug in dcbz
Message-ID: <20190730193502.GR31406@gate.crashing.org>
References: <20190729202542.205309-1-ndesaulniers@google.com> <20190729203246.GA117371@archlinux-threadripper> <20190729215200.GN31406@gate.crashing.org> <CAK8P3a1GQSyCj1L8fFG4Pah8dr5Lanw=1yuimX1o+53ARzOX+Q@mail.gmail.com> <20190730134856.GO31406@gate.crashing.org> <CAK8P3a2755_6xq453C2AePLW8BeQk_Jg=HfjB_F-zyVMnQDfdg@mail.gmail.com> <20190730161637.GP31406@gate.crashing.org> <CAK8P3a0_ovcX9tOo1UQ3_1UmM=+A2X=yErw27i2pHOj4XD40-A@mail.gmail.com>
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On Tue, Jul 30, 2019 at 08:24:14PM +0200, Arnd Bergmann wrote:
> On Tue, Jul 30, 2019 at 6:16 PM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > in_le32 and friends?  Yeah, huh.  If LLVM copies that to the stack as
> > well, its (not byte reversing) read will be atomic just fine, so things
> > will still work correctly.
> 
> byteorder is fine, the problem I was thinking of is when moving the load/store
> instructions around the barriers that synchronize with DMA, or turning
> them into different-size accesses. Changing two consecutive 16-bit mmio reads
> into an unaligned 32-bit read will rarely have the intended effect ;-)

Most such barriers will also work on the copy accesses, I think.  But
yes it depends on exactly how it is written.  The {in,out}_{be,le}<N>
ones use sync;store for out and sync;load;trap;isync for in, so they
should be safe ;-)

(Well, almost -- writes to I/O will not necessarily actually happen
before other stores, not from these macros alone at least).

Should be pretty easy to check what LLVM makes of this?


Segher