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

[Pavel Dovgalyuk]

> From: Markus Armbruster [mailto:address@hidden
> 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.

Thanks, I'll split this into two functions.

> 
> > +
> > +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?

In icount mode iothread and vcpu thread use the replay mutex for
serializing their operations. Therefore such races are impossible.

> 
> > +            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)

Ok.

> > +        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?

This is a doxygen-style comment for automatic documentation generation.

Pavel Dovgalyuk