14.6.3. 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:

- 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

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.

Probably you can step over the "rfi" instruction when using the BDI2000's telnet command interface instead of GDB.

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).

To debug the Linux kernel, I recommend that you run to a point where the MMU is on before you connect with GDB.

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.