From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
x86@kernel.org,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Denys Vlasenko <dvlasenk@redhat.com>
Subject: Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings
Date: Thu, 2 Mar 2017 06:49:26 -0600 [thread overview]
Message-ID: <20170302124926.47vfba4q6igkirzj@treble> (raw)
In-Reply-To: <20170302063139.GA594@gmail.com>
On Thu, Mar 02, 2017 at 07:31:39AM +0100, Ingo Molnar wrote:
>
> * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> > On Wed, Mar 01, 2017 at 11:42:54PM +0100, Arnd Bergmann wrote:
> > > On Wed, Mar 1, 2017 at 5:53 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > > > On Wed, Mar 01, 2017 at 04:27:29PM +0100, Arnd Bergmann wrote:
> > >
> > > > I see no apparent reason for the ud2.
> > >
> > > It's the possible division by zero. This change would avoid the ud2:
> > >
> > > diff --git a/drivers/i2c/busses/i2c-img-scb.c b/drivers/i2c/busses/i2c-img-scb.c
> > > index db8e8b40569d..a2b09c518225 100644
> > > --- a/drivers/i2c/busses/i2c-img-scb.c
> > > +++ b/drivers/i2c/busses/i2c-img-scb.c
> > > @@ -1196,6 +1196,8 @@ static int img_i2c_init(struct img_i2c *i2c)
> > > clk_khz /= prescale;
> > >
> > > /* Setup the clock increment value */
> > > + if (clk_khz < 1)
> > > + clk_khz = 1;
> > > inc = (256 * 16 * bitrate_khz) / clk_khz;
> > >
> > > /*
> >
> > Ok, I see what gcc is doing.
> >
> > clk_khz = clk_get_rate(i2c->scb_clk) / 1000;
> > ...
> > inc = (256 * 16 * bitrate_khz) / clk_khz;
> >
> > Because CONFIG_HAVE_CLK isn't set, clk_get_rate() returns 0, which means
> > clk_khz is always zero, so the last statement *always* results in a
> > divide-by-zero. So that looks like a bug in the code.
> >
> > However, I'm baffled by how gcc handles it. Instead of:
> >
> > a) reporting a compile-time warning/error; or
> >
> > b) letting the #DE (divide error) exception happen;
> >
> > it inserts a 'ud2', resulting in a #UD (invalid opcode). Why?!?
>
> Well, technically an invalid opcode is shorter code than generating an (integer)
> division by zero exception, right?
What does that matter if it's the wrong behavior?
--
Josh
next prev parent reply other threads:[~2017-03-02 14:07 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-10 12:56 [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings Arnd Bergmann
2016-10-10 20:23 ` Josh Poimboeuf
2016-10-11 8:08 ` Arnd Bergmann
2016-10-11 12:20 ` Josh Poimboeuf
2016-10-11 13:30 ` Arnd Bergmann
2016-10-11 15:05 ` Josh Poimboeuf
2016-10-11 15:51 ` Josh Poimboeuf
2016-10-11 20:38 ` Arnd Bergmann
2016-10-12 13:01 ` Josh Poimboeuf
2016-10-13 12:46 ` Another gcc corruption bug (was Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings) Josh Poimboeuf
2016-10-13 17:57 ` Denys Vlasenko
2016-10-13 20:15 ` Josh Poimboeuf
2017-03-01 9:34 ` [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings Arnd Bergmann
2017-03-01 9:45 ` Arnd Bergmann
2017-03-01 14:40 ` Josh Poimboeuf
2017-03-01 15:27 ` Arnd Bergmann
2017-03-01 16:53 ` Josh Poimboeuf
2017-03-01 22:05 ` Arnd Bergmann
2017-03-01 22:42 ` Arnd Bergmann
2017-03-02 1:03 ` Josh Poimboeuf
2017-03-02 6:31 ` Ingo Molnar
2017-03-02 12:49 ` Josh Poimboeuf [this message]
2017-03-02 13:46 ` Ingo Molnar
2017-03-02 14:08 ` Josh Poimboeuf
2017-03-02 14:46 ` Ingo Molnar
2017-03-02 22:49 ` Arnd Bergmann
2017-03-02 23:05 ` Josh Poimboeuf
2017-03-03 8:58 ` Arnd Bergmann
2017-03-03 11:27 ` Arnd Bergmann
2017-03-01 14:31 ` Josh Poimboeuf
2017-03-01 15:21 ` Arnd Bergmann
2017-03-02 18:25 ` Josh Poimboeuf
2017-03-02 22:43 ` Arnd Bergmann
2017-03-02 22:57 ` [PATCH] objtool: fix another gcc jump table detection issue Josh Poimboeuf
2017-03-02 23:01 ` Arnd Bergmann
2016-10-11 1:53 ` [PATCH] objtool: support '-mtune=atom' stack frame setup instruction Josh Poimboeuf
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