Re: [PATCH v5 14/15] replay: create temporary snapshot at debugger connection

[Philippe Mathieu-Daudé]

Hi Pavel,

On 9/22/20 2:16 PM, Pavel Dovgalyuk wrote:
> When record/replay does not uses overlays for storing the snapshots,
> user is not capable of issuing reverse debugging commands.
> This patch adds creation of the VM snapshot on the temporary
> overlay image, when the debugger connects to QEMU.
> Therefore the execution can be rewind to the moment
> of the debugger connection while debugging the virtual machine.
> 
> Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
> ---
>  gdbstub.c                 |    1 +
>  include/sysemu/replay.h   |    2 ++
>  replay/replay-debugging.c |   16 ++++++++++++++++
>  3 files changed, 19 insertions(+)
> 
> diff --git a/gdbstub.c b/gdbstub.c
> index ac92273018..f19f98ab1a 100644
> --- a/gdbstub.c
> +++ b/gdbstub.c
> @@ -3321,6 +3321,7 @@ static void gdb_chr_event(void *opaque, QEMUChrEvent 
> event)
>          s->g_cpu = s->c_cpu;
>  
>          vm_stop(RUN_STATE_PAUSED);
> +        replay_gdb_attached();
>          gdb_has_xml = false;
>          break;
>      default:
> diff --git a/include/sysemu/replay.h b/include/sysemu/replay.h
> index b6cac175c4..2aa34b8919 100644
> --- a/include/sysemu/replay.h
> +++ b/include/sysemu/replay.h
> @@ -94,6 +94,8 @@ bool replay_reverse_continue(void);
>  bool replay_running_debug(void);
>  /* Called in reverse debugging mode to collect breakpoint information */
>  void replay_breakpoint(void);
> +/* Called when gdb is attached to gdbstub */
> +void replay_gdb_attached(void);
>  
>  /* Processing the instructions */
>  
> diff --git a/replay/replay-debugging.c b/replay/replay-debugging.c
> index d02d4e0766..bb9110707a 100644
> --- a/replay/replay-debugging.c
> +++ b/replay/replay-debugging.c
> @@ -316,3 +316,19 @@ void replay_breakpoint(void)
>      assert(replay_mode == REPLAY_MODE_PLAY);
>      replay_last_breakpoint = replay_get_current_icount();
>  }
> +
> +void replay_gdb_attached(void)
> +{
> +    /*
> +     * Create VM snapshot on temporary overlay to allow reverse
> +     * debugging even if snapshots were not enabled.
> +     */
> +    if (replay_mode == REPLAY_MODE_PLAY
> +        && !replay_snapshot) {
> +        Error *err = NULL;
> +        if (save_snapshot("start_debugging", &err) != 0) {
> +            /* Can't create the snapshot. Continue conventional debugging. */

If you deliberately ignore the error, then use NULL;
else display the error.

> +            error_free(err);
> +        }
> +    }
> +}
>