[Qemu-devel] [Bug 181951] Re: Single-stepping PPC targets chokes on branch instructions

[Aurelien Jarno]

** Changed in: qemu
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/181951

Title:
  Single-stepping PPC targets chokes on branch instructions

Status in QEMU:
  Fix Released

Bug description:
  qemu CVS HEAD (11.01.2008)
  qemu-0.9.1 stable

  Single stepping (MSR_SE bit set in MSR) in qemu-system-ppc doesn't
  allow to step any branch instruction. Can be reproduced by boot a
  Linux image with a gdb something debuggable in the rootfs:

     / # gdb testprg
    GNU gdb 6.3.50.20050810
    Copyright 2004 Free Software Foundation, Inc.
    GDB is free software, covered by the GNU General Public License, and you are
    welcome to change it and/or distribute copies of it under certain 
conditions.
    Type "show copying" to see the conditions.
    There is absolutely no warranty for GDB.  Type "show warranty" for details.
    This GDB was configured as "powerpc-linux"...Using host libthread_db library
    "/lib/libthread_db.so.1".
    
    (gdb) b main
    Breakpoint 1 at 0x10000520: file testprg.c, line 26.
    (gdb) run
    Starting program: testprg
    Breakpoint 1, main () at testprg.c:26
    26  testprg.c: No such file or directory.
    in testprg.c
    (gdb) disassemble
    Dump of assembler code for function main:
    0x1000050c <main+0>:stwu    r1,-32(r1)
    0x10000510 <main+4>:mflr    r0
    0x10000514 <main+8>:stw     r31,28(r1)
    0x10000518 <main+12>:stw     r0,36(r1)
    0x1000051c <main+16>:mr      r31,r1
    0x10000520 <main+20>:lis     r9,4096
    0x10000524 <main+24>:addi    r3,r9,2376
    0x10000528 <main+28>:crclr   4*cr1+eq
    0x1000052c <main+32>:bl      0x10010ad8 <printf>
    0x10000530 <main+36>:lis     r9,4096
    ...
    (gdb) stepi
    0x10000524   26 in testprg.c
    (gdb) stepi
    0x10000528   26 in testprg.c
    (gdb) stepi
    0x1000052c   26 in testprg.c
    (gdb) stepi
    <<< QEMU HANGS! >>>

  The issue can be resolved using this patch:

  Index: target-ppc/translate.c
  ===================================================================
  RCS file: /sources/qemu/qemu/target-ppc/translate.c,v
  retrieving revision 1.115
  diff -u -r1.115 translate.c
  --- target-ppc/translate.c      24 Nov 2007 02:03:55 -0000      1.115
  +++ target-ppc/translate.c      10 Jan 2008 13:54:36 -0000
  @@ -2811,8 +2811,6 @@
   #endif
               gen_op_b_T1();
           gen_op_set_T0((long)tb + n);
  -        if (ctx->singlestep_enabled)
  -            gen_op_debug();
           gen_op_exit_tb();
       } else {
           gen_set_T1(dest);
  @@ -2823,8 +2821,6 @@
   #endif
               gen_op_b_T1();
           gen_op_reset_T0();
  -        if (ctx->singlestep_enabled)
  -            gen_op_debug();
           gen_op_exit_tb();
       }
   }
  @@ -3007,8 +3003,6 @@
               gen_op_btest_T1(ctx->nip);
           gen_op_reset_T0();
       no_test:
  -        if (ctx->singlestep_enabled)
  -            gen_op_debug();
           gen_op_exit_tb();
       }
    out: