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Date:   Thu, 23 Feb 2017 07:09:05 -0800
From:   hpa@...or.com
To:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
CC:     linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        arjan@...ux.intel.com, bp@...en8.de, jpoimboe@...hat.com,
        richard.weinberger@...il.com
Subject: Re: [PATCH] x86: Implement __WARN using UD0

On February 23, 2017 6:09:29 AM PST, Peter Zijlstra <peterz@...radead.org> 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?

Well, it only matters if the instruction extends past a segment boundary or page.  However, the CPU instruction decoder will consume a modrm for UD1, and so using just the two opcode bytes may cause a #PF or #GP when a #UD was intended.

This was documented very recently and Nasm hasn't caught up yet (hence the UNDOC flag.)
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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