From: Nick Desaulniers <ndesaulniers@google.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: James Y Knight <jyknight@google.com>,
LKML <linux-kernel@vger.kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>,
Paul Mackerras <paulus@samba.org>, Joel Stanley <joel@jms.id.au>,
Nathan Chancellor <natechancellor@gmail.com>,
linuxppc-dev@lists.ozlabs.org, dja@axtens.net
Subject: Re: [PATCH v2] powerpc: slightly improve cache helpers
Date: Thu, 25 Jul 2019 14:30:51 -0700 [thread overview]
Message-ID: <CAKwvOdkzBt=tTk+26dp+QsCStMUJ0_v5Mpjy2TOXPw1mu71itg@mail.gmail.com> (raw)
In-Reply-To: <20190722175817.GE20882@gate.crashing.org>
On Mon, Jul 22, 2019 at 10:58 AM Segher Boessenkool
<segher@kernel.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@kernel.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.
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2019-07-25 21:33 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-10 9:24 [PATCH v2] powerpc: slightly improve cache helpers Christophe Leroy
2019-07-08 1:19 ` Michael Ellerman
2019-07-08 19:14 ` Nathan Chancellor
2019-07-09 5:04 ` Christophe Leroy
2019-07-09 6:49 ` Nathan Chancellor
2019-07-19 3:24 ` Nathan Chancellor
2019-07-19 15:23 ` Segher Boessenkool
2019-07-19 16:04 ` Nathan Chancellor
2019-07-21 7:58 ` Nathan Chancellor
2019-07-21 18:01 ` Segher Boessenkool
2019-07-22 2:41 ` Nathan Chancellor
2019-07-22 6:19 ` Segher Boessenkool
2019-07-22 17:21 ` Nick Desaulniers
2019-07-22 17:58 ` Segher Boessenkool
2019-07-25 21:30 ` Nick Desaulniers [this message]
2019-07-29 20:28 ` Nick Desaulniers
2019-07-22 10:15 ` Michael Ellerman
2019-07-22 15:18 ` Segher Boessenkool
2019-07-22 23:21 ` Michael Ellerman
2019-07-25 13:22 ` Segher Boessenkool
2019-07-22 0:53 ` [PATCH] powerpc: Test broken dcbz kbuild test robot
2019-07-09 13:35 ` [PATCH v2] powerpc: slightly improve cache helpers Segher Boessenkool
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