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

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 20 Feb 2020 at 15:59, Changbin Du <changbin.du@gmail.com> 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
>
> Signed-off-by: Changbin Du <changbin.du@gmail.com>

Certainly if we support vCont we should advertise it. But why
does the fallback path not work? That is, why does gdb get a
wrong PC value initially?

thanks
-- PMM