linux-kernel - Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings

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Date:   Thu, 2 Mar 2017 07:31:39 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings


* Josh Poimboeuf <jpoimboe@...hat.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@...hat.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?

Thanks,

	Ingo