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Re: [PATCH] tcg: gdbstub: Fix single-step issue on arm target

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH] tcg: gdbstub: Fix single-step issue on arm target
Date: Thu, 20 Feb 2020 19:55:08 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 2/20/20 7:06 PM, Laurent Vivier wrote:
Le 20/02/2020 à 18:47, Philippe Mathieu-Daudé a écrit :
On 2/20/20 4:58 PM, Changbin Du wrote:
Recently when debugging an arm32 system on qemu, I found sometimes the
single-step command (stepi) is not working. This can be reproduced by
below steps:
   1) start qemu-system-arm -s -S .. and wait for gdb connection.
   2) start gdb and connect to qemu. In my case, gdb gets a wrong value
      (0x60) for PC.
   3) After connected, type 'stepi' and expect it will stop at next ins.

But, it has never stopped. This because:
   1) We doesn't report ‘vContSupported’ feature to gdb explicitly and gdb
      think we do not support it. In this case, gdb use a software
      to emulate single-step.
   2) Since gdb gets a wrong initial value of PC, then gdb inserts a
      breakpoint to wrong place (PC+4).

Since we do support ‘vContSupported’ query command, so let's tell gdb
we support it.

Before this change, gdb send below 'Z0' packet to implement single-step:
gdb_handle_packet: Z0,4,4

After this change, gdb send "vCont;s.." which is expected:
gdb_handle_packet: vCont?
put_packet: vCont;c;C;s;S
gdb_handle_packet: vCont;s:p1.1;c:p1.-1

You actually fixed this for all architectures :)

This has been annoying me on MIPS since more than a year...

The problem started with an update of QEMU or of GDB?

At one point it seemed to work, so what happened?

I'd say gdb. I can try different combinations of QEMU/gdb but I won't do that soon.

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