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

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH] tcg: gdbstub: Fix single-step issue on arm target
Date: Thu, 20 Feb 2020 18:47:26 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

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>

Signed-off-by: Changbin Du <address@hidden>
  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), 
      put_packet(gdb_ctx->s, gdb_ctx->str_buf);

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