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Message-ID: <CAKwvOd=knc1i+RBzJ2YTVKn3W3EzCeEnCfy3983WE6L5-V4x8A@mail.gmail.com>
Date: Mon, 29 Jul 2019 13:28:04 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Segher Boessenkool <segher@...nel.crashing.org>
Cc: Nathan Chancellor <natechancellor@...il.com>,
Christophe Leroy <christophe.leroy@....fr>,
Michael Ellerman <mpe@...erman.id.au>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Paul Mackerras <paulus@...ba.org>,
linuxppc-dev@...ts.ozlabs.org, LKML <linux-kernel@...r.kernel.org>,
clang-built-linux <clang-built-linux@...glegroups.com>,
James Y Knight <jyknight@...gle.com>,
Joel Stanley <joel@....id.au>, dja@...ens.net
Subject: Re: [PATCH v2] powerpc: slightly improve cache helpers
On Thu, Jul 25, 2019 at 2:30 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Mon, Jul 22, 2019 at 10:58 AM Segher Boessenkool
> <segher@...nel.crashing.org> wrote:
> >
> > On Mon, Jul 22, 2019 at 10:21:07AM -0700, Nick Desaulniers wrote:
> > > On Sun, Jul 21, 2019 at 11:19 PM Segher Boessenkool
> > > <segher@...nel.crashing.org> wrote:
> > > > On Sun, Jul 21, 2019 at 07:41:40PM -0700, Nathan Chancellor wrote:
> > > > > 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?
> >
> > It prints a memory address for an indexed operand.
> >
> > If you write just "%0" it prints addresses that are a single register
> > as "0(r3)" instead of "0,r3". Some instructions do not allow offset
> > form.
> >
> > > addr is in the list of
> > > inputs, so what's wrong with using it as an rvalue?
> >
> > It seems to use *(u8 *)addr as rvalue. Asm operands are lvalues. It
> > matters a lot for memory operands.
>
> Hmm...not sure that's specified behavior. Anyways, I've filed:
> https://bugs.llvm.org/show_bug.cgi?id=42762
> to see if folks more familiar with LLVM's ppc backend have some more thoughts.
>
> I recommend considering reverting commit 6c5875843b87 ("powerpc:
> slightly improve cache helpers") until the issue is resolved in clang,
> otherwise I'll probably just turn off our CI builds of PPC32 for the
> time being.
Started a new thread: https://lkml.org/lkml/2019/7/29/1483
--
Thanks,
~Nick Desaulniers
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