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

[Laurent Vivier]

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
>> breakpoint
>>      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
>> that
>> 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?

Thanks,
Laurent