Re: [Qemu-devel] [PATCH 2/2] qom: Implement qom-get HMP command

[Markus Armbruster <armbru@redhat.com>]

"Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> writes:

> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>
> Reimplement it based on qmp_qom_get() to avoid converting QObjects back
> to strings.

It's not using qmp_qom_get(), though.  That's a sign our abstractions
aren't quite right.  HMP command handlers should be implemented as
mostly trivial wrappers around the QMP handlers, or the two handlers
should be mostly trivial wrappers around a common helper function.  For
a more detailed explanation, see below.

Hmm, existing hmp_qom_set() has the same issue.  I'm not demanding you
clean this up.

> Inspired-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Andreas Färber <afaerber@suse.de>
>
> Slight fix for bit-rot:
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  hmp-commands.hx | 13 +++++++++++++
>  hmp.c           | 23 +++++++++++++++++++++++
>  hmp.h           |  1 +
>  3 files changed, 37 insertions(+)
>
> diff --git a/hmp-commands.hx b/hmp-commands.hx
> index 848efee..73f0372 100644
> --- a/hmp-commands.hx
> +++ b/hmp-commands.hx
> @@ -1736,6 +1736,19 @@ Print QOM properties of object at location @var{path}
>  ETEXI
>  
>      {
> +        .name       = "qom-get",
> +        .args_type  = "path:s,property:s",
> +        .params     = "path property",
> +        .help       = "print QOM property",
> +        .mhandler.cmd  = hmp_qom_get,
> +    },
> +
> +STEXI
> +@item qom-get @var{path} @var{property}
> +Print QOM property @var{property} of object at location @var{path}

This is a bit terse, but existing qom-list and qom-set are no less
terse.  The QAPI schema has more detail, perhaps we can adapt it for the
manual.  Not a blocker for me.

> +ETEXI
> +
> +    {
>          .name       = "qom-set",
>          .args_type  = "path:s,property:s,value:s",
>          .params     = "path property value",
> diff --git a/hmp.c b/hmp.c
> index cc2056e..17e6b06 100644
> --- a/hmp.c
> +++ b/hmp.c
> @@ -2064,6 +2064,29 @@ void hmp_qom_list(Monitor *mon, const QDict *qdict)
>      hmp_handle_error(mon, &err);
>  }
>  
> +void hmp_qom_get(Monitor *mon, const QDict *qdict)
> +{
> +    const char *path = qdict_get_str(qdict, "path");
> +    const char *property = qdict_get_str(qdict, "property");
> +    Error *err = NULL;
> +    Object *obj;
> +    char *value;
> +
> +    obj = object_resolve_path(path, NULL);
> +    if (obj == NULL) {
> +        error_set(&err, ERROR_CLASS_DEVICE_NOT_FOUND,
> +                  "Device '%s' not found", path);
> +        hmp_handle_error(mon, &err);
> +        return;
> +    }
> +    value = object_property_print(obj, property, true, &err);
> +    if (err == NULL) {
> +        monitor_printf(mon, "%s\n", value);
> +        g_free(value);
> +    }
> +    hmp_handle_error(mon, &err);
> +}
> +

Compare:

   QObject *qmp_qom_get(const char *path, const char *property, Error **errp)
   {
       Object *obj;

       obj = object_resolve_path(path, NULL);
       if (!obj) {
           error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
                     "Device '%s' not found", path);
           return NULL;
       }

       return object_property_get_qobject(obj, property, errp);
   }

The missing piece seems to be a function that prints the result of
object_property_get_qobject().  Since converting to QObject just for
printing is stupid, a common helper function seems more appropriate.
qmp_qom_get() would pass it the QMP output visitor to get a QObject, and
hmp_qom_get() would pass it the string output visitor to get a string.

>  void hmp_qom_set(Monitor *mon, const QDict *qdict)
>  {
>      const char *path = qdict_get_str(qdict, "path");
> diff --git a/hmp.h b/hmp.h
> index 0876ec0..882f339 100644
> --- a/hmp.h
> +++ b/hmp.h
> @@ -103,6 +103,7 @@ void hmp_object_del(Monitor *mon, const QDict *qdict);
>  void hmp_info_memdev(Monitor *mon, const QDict *qdict);
>  void hmp_info_memory_devices(Monitor *mon, const QDict *qdict);
>  void hmp_qom_list(Monitor *mon, const QDict *qdict);
> +void hmp_qom_get(Monitor *mon, const QDict *qdict);
>  void hmp_qom_set(Monitor *mon, const QDict *qdict);
>  void object_add_completion(ReadLineState *rs, int nb_args, const char *str);
>  void object_del_completion(ReadLineState *rs, int nb_args, const char *str);