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List:       opensolaris-dtrace-discuss
Subject:    [dtrace-discuss] NOP: 1 pid provider: 0
From:       James McIlree <jmcilree () apple ! com>
Date:       2006-12-30 15:13:53
Message-ID: 66F51E23-AA2B-471D-BC1C-7F922D8600E7 () apple ! com
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	I've been working on a project recently that uses multi-byte
NOPs. When I try to use the pid provider against that project, it  
refuses
to instrument the NOPs.

	I did some investigation and believe I have found the reason.
When fasttrap_tracepoint_init() runs, it attempts to find the size
of each instruction before installing a trap.

	The instruction decoder does not have an entry for the multi-byte
NOPs, and returns a bogus size. This causes the trap install to fail.

	I tested the following change:

m206-90:~/Projects/solaris/b50/usr/src/common/dis/i386> diff  
dis_tables.c dis_tables-fixed.c
815c815
< /*  [1C]  */  INVALID,                INVALID,                 
INVALID,       INVALID,
---
 > /*  [1C]  */  INVALID,                INVALID,                 
INVALID,       TS("nop",Mw),

	Which seem to fix the issue for me.

	Intel introduced the multi byte NOPs fairly recently, they are
documented in "IA-32 Intel® Architecture Software Developer’s Manual
Volume 2B", page 4 of chapter 4. My understanding is that these work
on AMD cpu's as well, but I have not been able to test that.

	I used the following method for testing, you may find it useful
as well (this is gcc based, not sure how Sun's cc will do with it):

void nop() {
         // 1 byte
         asm volatile(".byte 0x90" "\n\t");
         // 2 byte
         asm volatile(".byte 0x66, 0x90" "\n\t");
         // 3 byte
         asm volatile(".byte 0x0F, 0x1f, 0x00" "\n\t");
         // 4 byte
         asm volatile(".byte 0x0F, 0x1f, 0x40, 0x00" "\n\t");
         // 5 byte
         asm volatile(".byte 0x0F, 0x1f, 0x44, 0x00, 0x00" "\n\t");
         // 6 byte
         asm volatile(".byte 0x66, 0x0F, 0x1f, 0x44, 0x00, 0x00" "\n 
\t");
         // 7 byte
         asm volatile(".byte 0x0F, 0x1f, 0x80, 0x00, 0x00, 0x00, 0x00"  
"\n\t");
         // 8 byte
         asm volatile(".byte 0x0F, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00,  
0x00" "\n\t");
         // 9 bytes
         asm volatile(".byte 0x66, 0x0F, 0x1f, 0x84, 0x00, 0x00, 0x00,  
0x00, 0x00" "\n\t");
}


	James M

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