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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

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 <philmd@redhat.com>

> 
> Signed-off-by: Changbin Du <changbin.du@gmail.com>
> ---
>   gdbstub.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/gdbstub.c b/gdbstub.c
> index ce304ff482..adccd938e2 100644
> --- a/gdbstub.c
> +++ b/gdbstub.c
> @@ -2111,7 +2111,7 @@ static void handle_query_supported(GdbCmdContext *gdb_ctx, void *user_ctx)
>           gdb_ctx->s->multiprocess = true;
>       }
>   
> -    pstrcat(gdb_ctx->str_buf, sizeof(gdb_ctx->str_buf), ";multiprocess+");
> +    pstrcat(gdb_ctx->str_buf, sizeof(gdb_ctx->str_buf), ";vContSupported+;multiprocess+");
>       put_packet(gdb_ctx->s, gdb_ctx->str_buf);
>   }
>   
>