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Date: Thu, 23 Feb 2017 15:59:06 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        arjan@linux.intel.com, bp@alien8.de, jpoimboe@redhat.com,
        richard.weinberger@gmail.com
Subject: Re: [PATCH] x86: Implement __WARN using UD0
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On Thu, Feb 23, 2017 at 03:09:29PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 23, 2017 at 02:28:13PM +0100, Peter Zijlstra wrote:
> > + * Since various instruction decoders disagree on the length of UD1,
> > + * we cannot use it either. So use UD0 for WARN.
> > + *
> > + * (binutils knows about "ud1" but {en,de}codes it as 2 bytes, whereas
> > + *  our kernel decoder thinks it takes a ModRM byte, which seems consistent
> > + *  with various things like the Intel SDM instruction encoding rules)
> > + */
> > +
> > +#define ASM_UD0		".byte 0x0f, 0xff"
> > +#define ASM_UD1		".byte 0x0f, 0xb9" /* + ModRM */
> > +#define ASM_UD2		".byte 0x0f, 0x0b"
> 
> http://repo.or.cz/nasm.git/blob/HEAD:/x86/insns.dat
> 
> has:
> 
> 1378 UD0             void                            [       0f ff]                                  186,UNDOC
> 1379 UD1             void                            [       0f b9]                                  186,UNDOC
> 1380 UD2B            void                            [       0f b9]                                  186,UNDOC,ND
> 1381 UD2             void                            [       0f 0b]                                  186
> 1382 UD2A            void                            [       0f 0b]                                  186,ND
> 
> which seems to use the 2 byte version of UD1.
> 
> hpa, any input?

N/m, as Josh said, they're listed in the latest SDM (Dec 2016) and that
lists UD1 /r, so 3 bytes.

This means binutils decodes it wrong.