HowToSingleStepThroughRFIInstruction < DULG

%SECTION0{name=HowToSingleStepThroughRFIInstruction}% How to single step through "RFI" instruction

   Question:: I am trying to debug Linux on an IBM 405GP
    processor. Linux boots fine and I can step through the code until
    the ="rfi"= instruction in =head_4xx.S=; then I get the following:
      <verbatim>
- TARGET: target has entered debug mode
    Target state      : debug mode
    Debug entry cause : JTAG stop request
    Current PC        : 0x00000700
    Current CR        : 0x28004088
    Current MSR       : 0x00000000
    Current LR        : 0x000007a8
# Step timeout detected
      </verbatim>

   Answer:: Your single step problem most likely comes from
    the fact that GDB accesses some non-existent memory
    (at least some versions do/did in the past).
    This exception is stored in some way
    within the 405 and when you step ="rfi"= it triggers.
    This is because some instructions like ="rfi"= are always
    stepped using a hardware breakpoint and not with
    the JTAG single step feature.
    <p></p>
    Probably you can step over the ="rfi"= instruction when using the
    BDI2000's =telnet= command interface instead of GDB.
    <p></p>
    Similar problems have also been reported when stepping through
    ="mtmsr"= or ="mfmsr"= during initial boot code.
    The problem comes also from the fact that GDB accesses
    non-existent memory (maybe it tries to read a non-existent stack frame).
    <p></p>
    To debug the Linux kernel, I recommend that you run to a point
    where the MMU is on before you connect with GDB.
    <p></p>
    To debug boot code where the MMU is off I recommend to use
    the MMAP feature of the BDI to prevent illegal memory
    accesses from GDB.
14.6.2. How to Debug Linux Exceptions		1. Abstract		14.6.4. Setting a breakpoint doesn't work
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