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

[Changbin Du]

On Thu, Feb 20, 2020 at 06:47:26PM +0100, Philippe Mathieu-Daudé wrote:
> 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...
> 
> I haven't checked the GDB protocol spec, but so far:
> Tested-by: Philippe Mathieu-Daudé <address@hidden>
>
Thanks for your feedback. :)

-- 
Cheers,
Changbin Du