gdb for Riscv, single stepping issue

John Baldwin
Tue Jun 28 15:52:20 GMT 2022

On 6/27/22 1:40 PM, James Becker wrote:
> Hello,
> I have a RISCV-EL2 core running in a Nexys A7 FPGA board.
> I have openocd for riscv running over jtag with a connection by
> riscv-gdb to the openocd instance at port 3333.
> Everything works fine, stepping, break points, load, view memory.
> But I have one issue: Some of the memory in my design is 4 byte
> aligned.  Its designed for fast instruction fetch, its known as ICCM.
> When I have code running in that memory, gdb still works fine for
> breakpoints, but it will not single step.
> Looking at the openocd debug files, it appears that gdb is attempting to
> do a 2 byte read as a part of the single stepping procedure.
> Since my memory does not support 2 byte reads or writes, this fails.
> Is there some way that gdb can be configured to not do any 2-byte word
> reads or writes during single stepping?  I can't seem to find any.

Hmm, setting breakpoints tries to read 1 byte at a time unless you have
disabled compressed breakpoints.  It looks like as a local hack you could
change use-compressed-breakpoints to just read 4 bytes rather than 2
initially?  Perhaps this is upstreamable if you make it read 4 bytes if
the target address is 4 byte aligned and only fall back to reading 2 bytes
if it isn't?

John Baldwin

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