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

[Philippe Mathieu-Daudé]

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
> > > 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?

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