Re: [Qemu-devel] [PATCH v6 10/25] replay: introduce breakpoint at the specified step

[Markus Armbruster]

Pavel Dovgalyuk <address@hidden> writes:

> This patch introduces replay_break qmp and hmp commands.
> These commands allow stopping at the specified instruction.
> It may be useful for debugging when there are some known
> events that should be investigated.
> The commands have one argument - number of instructions
> executed since the start of the replay.
>
> Signed-off-by: Pavel Dovgalyuk <address@hidden>
>
> --
>
> v2:
>  - renamed replay_break qmp command into replay-break
>    (suggested by Eric Blake)
> ---
>  hmp-commands.hx           |   15 ++++++++++++
>  hmp.h                     |    1 +
>  include/sysemu/replay.h   |    3 ++
>  qapi/misc.json            |   17 ++++++++++++++
>  replay/replay-debugging.c |   55 
> +++++++++++++++++++++++++++++++++++++++++++++
>  replay/replay-internal.h  |    4 +++
>  replay/replay.c           |   17 ++++++++++++++
>  7 files changed, 112 insertions(+)
>
> diff --git a/hmp-commands.hx b/hmp-commands.hx
> index db0c681..aa0794e 100644
> --- a/hmp-commands.hx
> +++ b/hmp-commands.hx
> @@ -1888,6 +1888,21 @@ Set QOM property @var{property} of object at location 
> @var{path} to value @var{v
>  ETEXI
>  
>      {
> +        .name       = "replay_break",
> +        .args_type  = "step:i",
> +        .params     = "step",
> +        .help       = "sets breakpoint on the specified step of the replay",
> +        .cmd        = hmp_replay_break,
> +    },
> +
> +STEXI
> address@hidden replay_break @var{step}
> address@hidden replay_break
> +Set breakpoint on the specified step of the replay.
> +Execution stops when the specified step is reached.
> +ETEXI
> +
> +    {
>          .name       = "info",
>          .args_type  = "item:s?",
>          .params     = "[subcommand]",
> diff --git a/hmp.h b/hmp.h
> index d792149..ad94abe 100644
> --- a/hmp.h
> +++ b/hmp.h
> @@ -149,5 +149,6 @@ void hmp_info_vm_generation_id(Monitor *mon, const QDict 
> *qdict);
>  void hmp_info_memory_size_summary(Monitor *mon, const QDict *qdict);
>  void hmp_info_sev(Monitor *mon, const QDict *qdict);
>  void hmp_info_replay(Monitor *mon, const QDict *qdict);
> +void hmp_replay_break(Monitor *mon, const QDict *qdict);
>  
>  #endif
> diff --git a/include/sysemu/replay.h b/include/sysemu/replay.h
> index a5510f2..c9ba75a 100644
> --- a/include/sysemu/replay.h
> +++ b/include/sysemu/replay.h
> @@ -73,6 +73,9 @@ void replay_finish(void);
>  void replay_add_blocker(Error *reason);
>  /*! Returns name of the replay log file */
>  const char *replay_get_filename(void);
> +/*! Sets breakpoint at the specified step.
> +    If step = -1LL the existing breakpoint is removed. */
> +void replay_break(int64_t step, QEMUTimerCB callback, void *opaque);
>  
>  /* Processing the instructions */
>  
> diff --git a/qapi/misc.json b/qapi/misc.json
> index e246ce3..4fcd211 100644
> --- a/qapi/misc.json
> +++ b/qapi/misc.json
> @@ -3135,6 +3135,23 @@
>    'returns': 'ReplayInfo' }
>  
>  ##
> +# @replay-break:
> +#
> +# Set breakpoint on the specified step of the replay.
> +# Execution stops when the specified step is reached.
> +#
> +# @step: execution step to stop at
> +#
> +# Since: 3.1
> +#
> +# Example:
> +#
> +# -> { "execute": "replay-break", "data": { "step": 220414 } }
> +#
> +##
> +{ 'command': 'replay-break', 'data': { 'step': 'int' } }
> +
> +##
>  # @xen-load-devices-state:
>  #
>  # Load the state of all devices from file. The RAM and the block devices
> diff --git a/replay/replay-debugging.c b/replay/replay-debugging.c
> index 2d364fa..c6b80c0 100644
> --- a/replay/replay-debugging.c
> +++ b/replay/replay-debugging.c
> @@ -16,6 +16,8 @@
>  #include "hmp.h"
>  #include "monitor/monitor.h"
>  #include "qapi/qapi-commands-misc.h"
> +#include "qapi/qmp/qdict.h"
> +#include "qemu/timer.h"
>  
>  void hmp_info_replay(Monitor *mon, const QDict *qdict)
>  {
> @@ -39,3 +41,56 @@ ReplayInfo *qmp_query_replay(Error **errp)
>      retval->step = replay_get_current_step();
>      return retval;
>  }
> +
> +void replay_break(int64_t step, QEMUTimerCB callback, void *opaque)
> +{
> +    assert(replay_mode == REPLAY_MODE_PLAY);
> +    assert(replay_mutex_locked());
> +
> +    replay_break_step = step;
> +    if (replay_break_timer) {
> +        timer_del(replay_break_timer);
> +        timer_free(replay_break_timer);
> +        replay_break_timer = NULL;
> +    }
> +
> +    if (replay_break_step == -1LL) {
> +        return;
> +    }
> +    assert(replay_break_step >= replay_get_current_step());
> +    assert(callback);
> +
> +    replay_break_timer = timer_new_ns(QEMU_CLOCK_REALTIME, callback, opaque);
> +}

This function multiplexes

(a) deleting the breakpoint: @step is -1, @callback and @opaque are
    ignored

(b) setting the breakpoint: step must be >= replay_get_current_step()
    (which implies it's not -1), @callback must be non-null

I hate such multiplexing.  I'd be more willing to tolerate it for a
static function.  Why isn't it static?  I can't see uses outside this
file...

To avoid the multiplexing, you could duplicate the function, specialize
both copies, then factor out the common code into a helper function.

> +
> +static void replay_stop_vm(void *opaque)
> +{
> +    vm_stop(RUN_STATE_PAUSED);
> +    replay_break(-1LL, NULL, NULL);
> +}
> +
> +void qmp_replay_break(int64_t step, Error **errp)
> +{
> +    if (replay_mode ==  REPLAY_MODE_PLAY) {
> +        if (step >= replay_get_current_step()) {

Pardon another ignorant question: what ensures replay_get_current_step()
stays put until we completed setting the breakpoint?

> +            replay_break(step, replay_stop_vm, NULL);
> +        } else {
> +            error_setg(errp, "cannot set break at the step in the past");
> +        }

Duplicates the >= replay_get_current_step() check we just saw in
replay_break().  Consider

       if (replay_mode ==  REPLAY_MODE_PLAY) {
           replay_break(step, replay_stop_vm, NULL, errp);

and setting an error in replay_break().  Suggestion, not demand.

> +    } else {
> +        error_setg(errp, "setting the break is allowed only in play mode");

s/the break/the breakpoint/

> +    }
> +}
> +
> +void hmp_replay_break(Monitor *mon, const QDict *qdict)
> +{
> +    int64_t step = qdict_get_try_int(qdict, "step", -1LL);
> +    Error *err = NULL;
> +
> +    qmp_replay_break(step, &err);
> +    if (err) {
> +        monitor_printf(mon, "replay_break error: %s\n", 
> error_get_pretty(err));
> +        error_free(err);

There's no need for the "replay_break error: " prefix; the user already
knows what command he just issued.  Thus:

           error_report_err(err)

> +        return;
> +    }
> +}
> diff --git a/replay/replay-internal.h b/replay/replay-internal.h
> index 4f82676..f9cad9d 100644
> --- a/replay/replay-internal.h
> +++ b/replay/replay-internal.h
> @@ -91,6 +91,10 @@ extern ReplayState replay_state;
>  
>  /* File for replay writing */
>  extern FILE *replay_file;
> +/*! Step of the replay breakpoint */

Why the bang?

> +extern int64_t replay_break_step;
> +/*! Timer for the replay breakpoint callback */

Likewise.

> +extern QEMUTimer *replay_break_timer;
>  
>  void replay_put_byte(uint8_t byte);
>  void replay_put_event(uint8_t event);
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