diff mbox series

[v3,1/4] keyval: Parse help options

Message ID 20201007164903.282198-2-kwolf@redhat.com
State New, archived
Headers show
Series qemu-storage-daemon: Remove QemuOpts from --object parser | expand

Commit Message

Kevin Wolf Oct. 7, 2020, 4:49 p.m. UTC
This adds a special meaning for 'help' and '?' as options to the keyval
parser. Instead of being an error (because of a missing value) or a
value for an implied key, they now request help, which is a new boolean
ouput of the parser in addition to the QDict.

A new parameter 'p_help' is added to keyval_parse() that contains on
return whether help was requested. If NULL is passed, requesting help
results in an error and all other cases work like before.

Turning previous error cases into help is a compatible extension. The
behaviour potentially changes for implied keys: They could previously
get 'help' as their value, which is now interpreted as requesting help.

This is not a problem in practice because 'help' and '?' are not a valid
values for the implied key of any option parsed with keyval_parse():

* audiodev: union Audiodev, implied key "driver" is enum AudiodevDriver,
  "help" and "?" are not among its values

* display: union DisplayOptions, implied key "type" is enum
  DisplayType, "help" and "?" are not among its values

* blockdev: union BlockdevOptions, implied key "driver is enum
  BlockdevDriver, "help" and "?" are not among its values

* export: union BlockExport, implied key "type" is enum BlockExportType,
  "help" and "?" are not among its values

* monitor: struct MonitorOptions, implied key "mode"is enum MonitorMode,
  "help" and "?" are not among its values

* nbd-server: struct NbdServerOptions, no implied key.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 include/qemu/help_option.h           |   5 +
 include/qemu/option.h                |   2 +-
 qapi/qobject-input-visitor.c         |   2 +-
 storage-daemon/qemu-storage-daemon.c |   2 +-
 tests/test-keyval.c                  | 205 +++++++++++++++++++--------
 util/keyval.c                        |  54 ++++++-
 6 files changed, 198 insertions(+), 72 deletions(-)

Comments

Eric Blake Oct. 7, 2020, 5:29 p.m. UTC | #1
On 10/7/20 11:49 AM, Kevin Wolf wrote:
> This adds a special meaning for 'help' and '?' as options to the keyval
> parser. Instead of being an error (because of a missing value) or a
> value for an implied key, they now request help, which is a new boolean
> ouput of the parser in addition to the QDict.

output

> 
> A new parameter 'p_help' is added to keyval_parse() that contains on
> return whether help was requested. If NULL is passed, requesting help
> results in an error and all other cases work like before.
> 
> Turning previous error cases into help is a compatible extension. The
> behaviour potentially changes for implied keys: They could previously
> get 'help' as their value, which is now interpreted as requesting help.
> 
> This is not a problem in practice because 'help' and '?' are not a valid
> values for the implied key of any option parsed with keyval_parse():
> 
> * audiodev: union Audiodev, implied key "driver" is enum AudiodevDriver,
>   "help" and "?" are not among its values
> 
> * display: union DisplayOptions, implied key "type" is enum
>   DisplayType, "help" and "?" are not among its values
> 
> * blockdev: union BlockdevOptions, implied key "driver is enum
>   BlockdevDriver, "help" and "?" are not among its values
> 
> * export: union BlockExport, implied key "type" is enum BlockExportType,
>   "help" and "?" are not among its values
> 
> * monitor: struct MonitorOptions, implied key "mode"is enum MonitorMode,

missing space

>   "help" and "?" are not among its values
> 
> * nbd-server: struct NbdServerOptions, no implied key.

Including the audit is nice (and thanks to Markus for helping derive the
list).

> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---

> +++ b/util/keyval.c
> @@ -14,7 +14,7 @@
>   * KEY=VALUE,... syntax:
>   *
>   *   key-vals     = [ key-val { ',' key-val } [ ',' ] ]
> - *   key-val      = key '=' val
> + *   key-val      = 'help' | key '=' val

Maybe: = 'help' | '?' | key = '=' val

>   *   key          = key-fragment { '.' key-fragment }
>   *   key-fragment = / [^=,.]* /
>   *   val          = { / [^,]* / | ',,' }
> @@ -73,10 +73,14 @@
>   *
>   * Additional syntax for use with an implied key:
>   *
> - *   key-vals-ik  = val-no-key [ ',' key-vals ]
> + *   key-vals-ik  = 'help' | val-no-key [ ',' key-vals ]

and again for '?' here.  Actually, this should probably be:

key-vals-ik  = 'help' [ ',' key-vals ]
             | '?' [ ',' key-vals ]
             | val-no-key [ ',' key-vals ]

>   *   val-no-key   = / [^=,]* /

This is now slightly inaccurate, but I don't know how to concisely
express the regex [^=,]* but not '?' or 'help', and the prose covers the
ambiguity.


> @@ -410,6 +442,14 @@ QDict *keyval_parse(const char *params, const char *implied_key,
>          implied_key = NULL;
>      }
>  
> +    if (p_help) {
> +        *p_help = help;
> +    } else if (help) {
> +        error_setg(errp, "Help is not available for this option");
> +        qobject_unref(qdict);
> +        return NULL;
> +    }

This part is a definite improvement over v2.

I'm assuming Markus can help touch up the comments and spelling errors, so:

Reviewed-by: Eric Blake <eblake@redhat.com>
Kevin Wolf Oct. 8, 2020, 8:45 a.m. UTC | #2
Am 07.10.2020 um 19:29 hat Eric Blake geschrieben:
> On 10/7/20 11:49 AM, Kevin Wolf wrote:
> > This adds a special meaning for 'help' and '?' as options to the keyval
> > parser. Instead of being an error (because of a missing value) or a
> > value for an implied key, they now request help, which is a new boolean
> > ouput of the parser in addition to the QDict.
> 
> output
> 
> > 
> > A new parameter 'p_help' is added to keyval_parse() that contains on
> > return whether help was requested. If NULL is passed, requesting help
> > results in an error and all other cases work like before.
> > 
> > Turning previous error cases into help is a compatible extension. The
> > behaviour potentially changes for implied keys: They could previously
> > get 'help' as their value, which is now interpreted as requesting help.
> > 
> > This is not a problem in practice because 'help' and '?' are not a valid
> > values for the implied key of any option parsed with keyval_parse():
> > 
> > * audiodev: union Audiodev, implied key "driver" is enum AudiodevDriver,
> >   "help" and "?" are not among its values
> > 
> > * display: union DisplayOptions, implied key "type" is enum
> >   DisplayType, "help" and "?" are not among its values
> > 
> > * blockdev: union BlockdevOptions, implied key "driver is enum
> >   BlockdevDriver, "help" and "?" are not among its values
> > 
> > * export: union BlockExport, implied key "type" is enum BlockExportType,
> >   "help" and "?" are not among its values
> > 
> > * monitor: struct MonitorOptions, implied key "mode"is enum MonitorMode,
> 
> missing space
> 
> >   "help" and "?" are not among its values
> > 
> > * nbd-server: struct NbdServerOptions, no implied key.
> 
> Including the audit is nice (and thanks to Markus for helping derive the
> list).
> 
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> 
> > +++ b/util/keyval.c
> > @@ -14,7 +14,7 @@
> >   * KEY=VALUE,... syntax:
> >   *
> >   *   key-vals     = [ key-val { ',' key-val } [ ',' ] ]
> > - *   key-val      = key '=' val
> > + *   key-val      = 'help' | key '=' val
> 
> Maybe: = 'help' | '?' | key = '=' val
> 
> >   *   key          = key-fragment { '.' key-fragment }
> >   *   key-fragment = / [^=,.]* /
> >   *   val          = { / [^,]* / | ',,' }
> > @@ -73,10 +73,14 @@
> >   *
> >   * Additional syntax for use with an implied key:
> >   *
> > - *   key-vals-ik  = val-no-key [ ',' key-vals ]
> > + *   key-vals-ik  = 'help' | val-no-key [ ',' key-vals ]
> 
> and again for '?' here.

Ah, true, I should mention '?', too.

> Actually, this should probably be:
> 
> key-vals-ik  = 'help' [ ',' key-vals ]
>              | '?' [ ',' key-vals ]
>              | val-no-key [ ',' key-vals ]

I noticed that the grammar was wrong even before my patch (because
making use of the implied key is optional), but the right fix wasn't
obvious to me, so I decided to just leave that part as it is.

Even your version is wrong because it's valid to a value with implied
key and help at the same time.

Thinking a bit more about it, I feel it should simply be something like:

    key-vals-ik = val-no-key [ ',' key-vals ] | key-vals

And then key-vals automatically covers the help case.

> >   *   val-no-key   = / [^=,]* /
> 
> This is now slightly inaccurate, but I don't know how to concisely
> express the regex [^=,]* but not '?' or 'help', and the prose covers the
> ambiguity.
> 
> 
> > @@ -410,6 +442,14 @@ QDict *keyval_parse(const char *params, const char *implied_key,
> >          implied_key = NULL;
> >      }
> >  
> > +    if (p_help) {
> > +        *p_help = help;
> > +    } else if (help) {
> > +        error_setg(errp, "Help is not available for this option");
> > +        qobject_unref(qdict);
> > +        return NULL;
> > +    }
> 
> This part is a definite improvement over v2.
> 
> I'm assuming Markus can help touch up the comments and spelling errors, so:
> 
> Reviewed-by: Eric Blake <eblake@redhat.com>

I assumed that as a qsd series this would go through my own tree anyway,
so if all of you agree that you don't want to see the corrected version
on the list, I can fix them while applying the series.

Kevin
Markus Armbruster Oct. 8, 2020, 3:25 p.m. UTC | #3
Kevin Wolf <kwolf@redhat.com> writes:

> Am 07.10.2020 um 19:29 hat Eric Blake geschrieben:
>> On 10/7/20 11:49 AM, Kevin Wolf wrote:
>> > This adds a special meaning for 'help' and '?' as options to the keyval
>> > parser. Instead of being an error (because of a missing value) or a
>> > value for an implied key, they now request help, which is a new boolean
>> > ouput of the parser in addition to the QDict.
>> 
>> output
>> 
>> > 
>> > A new parameter 'p_help' is added to keyval_parse() that contains on
>> > return whether help was requested. If NULL is passed, requesting help
>> > results in an error and all other cases work like before.
>> > 
>> > Turning previous error cases into help is a compatible extension. The
>> > behaviour potentially changes for implied keys: They could previously
>> > get 'help' as their value, which is now interpreted as requesting help.
>> > 
>> > This is not a problem in practice because 'help' and '?' are not a valid
>> > values for the implied key of any option parsed with keyval_parse():
>> > 
>> > * audiodev: union Audiodev, implied key "driver" is enum AudiodevDriver,
>> >   "help" and "?" are not among its values
>> > 
>> > * display: union DisplayOptions, implied key "type" is enum
>> >   DisplayType, "help" and "?" are not among its values
>> > 
>> > * blockdev: union BlockdevOptions, implied key "driver is enum
>> >   BlockdevDriver, "help" and "?" are not among its values
>> > 
>> > * export: union BlockExport, implied key "type" is enum BlockExportType,
>> >   "help" and "?" are not among its values
>> > 
>> > * monitor: struct MonitorOptions, implied key "mode"is enum MonitorMode,
>> 
>> missing space
>> 
>> >   "help" and "?" are not among its values
>> > 
>> > * nbd-server: struct NbdServerOptions, no implied key.
>> 
>> Including the audit is nice (and thanks to Markus for helping derive the
>> list).
>> 
>> > 
>> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
>> > ---
>> 
>> > +++ b/util/keyval.c
>> > @@ -14,7 +14,7 @@
>> >   * KEY=VALUE,... syntax:
>> >   *
>> >   *   key-vals     = [ key-val { ',' key-val } [ ',' ] ]
>> > - *   key-val      = key '=' val
>> > + *   key-val      = 'help' | key '=' val
>> 
>> Maybe: = 'help' | '?' | key = '=' val
>> 
>> >   *   key          = key-fragment { '.' key-fragment }
>> >   *   key-fragment = / [^=,.]* /
>> >   *   val          = { / [^,]* / | ',,' }
>> > @@ -73,10 +73,14 @@
>> >   *
>> >   * Additional syntax for use with an implied key:
>> >   *
>> > - *   key-vals-ik  = val-no-key [ ',' key-vals ]
>> > + *   key-vals-ik  = 'help' | val-no-key [ ',' key-vals ]
>> 
>> and again for '?' here.
>
> Ah, true, I should mention '?', too.

Let's use a non-terminal

    help = 'help' | '?'
    key-val = key '=' val | help

>> Actually, this should probably be:
>> 
>> key-vals-ik  = 'help' [ ',' key-vals ]
>>              | '?' [ ',' key-vals ]
>>              | val-no-key [ ',' key-vals ]
>
> I noticed that the grammar was wrong even before my patch (because
> making use of the implied key is optional), but the right fix wasn't
> obvious to me, so I decided to just leave that part as it is.

The grammar leaves a small, but important unsaid, or rather said only in
the accompanying prose.

 * Additional syntax for use with an implied key:
 *
 *   key-vals-ik  = val-no-key [ ',' key-vals ]
 *   val-no-key   = / [^=,]* /

tries to express that with an implied key, the grammar changes from

     S = key-vals

to

     S = key-vals | key-vals-ik

See, "additional syntax".  Felt clear enough to me when I wrote it.  It
is not.

> Even your version is wrong because it's valid to a value with implied
> key and help at the same time.
>
> Thinking a bit more about it, I feel it should simply be something like:
>
>     key-vals-ik = val-no-key [ ',' key-vals ] | key-vals
>
> And then key-vals automatically covers the help case.

Unfortunately, this simple grammar is ambigious: "help" can be parsed
both as

    "help" -> help -> key-val -> key-vals -> key-vals-ik

and

    "help" -> val-no-key -> key-vals-ik

Ham-fisted fix:

    val-no-key = / [^-,]* / - 'help'

I'm not a fan of the exception operator.  Better ideas?

I'll have a closer look at the actual patch next.

>> >   *   val-no-key   = / [^=,]* /
>> 
>> This is now slightly inaccurate, but I don't know how to concisely
>> express the regex [^=,]* but not '?' or 'help', and the prose covers the
>> ambiguity.
>> 
>> 
>> > @@ -410,6 +442,14 @@ QDict *keyval_parse(const char *params, const char *implied_key,
>> >          implied_key = NULL;
>> >      }
>> >  
>> > +    if (p_help) {
>> > +        *p_help = help;
>> > +    } else if (help) {
>> > +        error_setg(errp, "Help is not available for this option");
>> > +        qobject_unref(qdict);
>> > +        return NULL;
>> > +    }
>> 
>> This part is a definite improvement over v2.
>> 
>> I'm assuming Markus can help touch up the comments and spelling errors, so:
>> 
>> Reviewed-by: Eric Blake <eblake@redhat.com>
>
> I assumed that as a qsd series this would go through my own tree anyway,
> so if all of you agree that you don't want to see the corrected version
> on the list, I can fix them while applying the series.

The series spans "Command line option argument parsing" (this patch),
"QOM" (next two), and qsd (final one).  I'm fine with you taking it
through your tree.

Just noticed: you neglected to cc: the QOM maintainers.  Recommend to
give them a heads-up.
Markus Armbruster Oct. 9, 2020, 3:10 p.m. UTC | #4
Kevin Wolf <kwolf@redhat.com> writes:

> This adds a special meaning for 'help' and '?' as options to the keyval
> parser. Instead of being an error (because of a missing value) or a
> value for an implied key, they now request help, which is a new boolean
> ouput of the parser in addition to the QDict.
>
> A new parameter 'p_help' is added to keyval_parse() that contains on
> return whether help was requested. If NULL is passed, requesting help
> results in an error and all other cases work like before.
>
> Turning previous error cases into help is a compatible extension. The
> behaviour potentially changes for implied keys: They could previously
> get 'help' as their value, which is now interpreted as requesting help.
>
> This is not a problem in practice because 'help' and '?' are not a valid
> values for the implied key of any option parsed with keyval_parse():
>
> * audiodev: union Audiodev, implied key "driver" is enum AudiodevDriver,
>   "help" and "?" are not among its values
>
> * display: union DisplayOptions, implied key "type" is enum
>   DisplayType, "help" and "?" are not among its values
>
> * blockdev: union BlockdevOptions, implied key "driver is enum
>   BlockdevDriver, "help" and "?" are not among its values
>
> * export: union BlockExport, implied key "type" is enum BlockExportType,
>   "help" and "?" are not among its values
>
> * monitor: struct MonitorOptions, implied key "mode"is enum MonitorMode,
>   "help" and "?" are not among its values
>
> * nbd-server: struct NbdServerOptions, no implied key.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  include/qemu/help_option.h           |   5 +
>  include/qemu/option.h                |   2 +-
>  qapi/qobject-input-visitor.c         |   2 +-
>  storage-daemon/qemu-storage-daemon.c |   2 +-
>  tests/test-keyval.c                  | 205 +++++++++++++++++++--------
>  util/keyval.c                        |  54 ++++++-
>  6 files changed, 198 insertions(+), 72 deletions(-)
>
> diff --git a/include/qemu/help_option.h b/include/qemu/help_option.h
> index 328d2a89fd..952d628b9d 100644
> --- a/include/qemu/help_option.h
> +++ b/include/qemu/help_option.h
> @@ -19,4 +19,9 @@ static inline bool is_help_option(const char *s)
>      return !strcmp(s, "?") || !strcmp(s, "help");
>  }
>  
> +static inline bool is_help_option_n(const char *s, size_t n)
> +{
> +    return !strncmp(s, "?", n) || !strncmp(s, "help", n);
> +}
> +
>  #endif

Unlike is_help_option(), this also recognizes prefixes of "help":

    $ upstream-qemu-storage-daemon --object secret,hel
    secret options:
      data=<string>
      file=<string>
      format=<QCryptoSecretFormat>
      iv=<string>
      keyid=<string>
      loaded=<bool>
    $ upstream-qemu-storage-daemon --object hel
    upstream-qemu-storage-daemon: Parameter 'id' is missing

> diff --git a/include/qemu/option.h b/include/qemu/option.h
> index 05e8a15c73..ac69352e0e 100644
> --- a/include/qemu/option.h
> +++ b/include/qemu/option.h
> @@ -149,6 +149,6 @@ void qemu_opts_free(QemuOptsList *list);
>  QemuOptsList *qemu_opts_append(QemuOptsList *dst, QemuOptsList *list);
>  
>  QDict *keyval_parse(const char *params, const char *implied_key,
> -                    Error **errp);
> +                    bool *help, Error **errp);
>  
>  #endif
> diff --git a/qapi/qobject-input-visitor.c b/qapi/qobject-input-visitor.c
> index f918a05e5f..7b184b50a7 100644
> --- a/qapi/qobject-input-visitor.c
> +++ b/qapi/qobject-input-visitor.c
> @@ -757,7 +757,7 @@ Visitor *qobject_input_visitor_new_str(const char *str,
>          assert(args);
>          v = qobject_input_visitor_new(QOBJECT(args));
>      } else {
> -        args = keyval_parse(str, implied_key, errp);
> +        args = keyval_parse(str, implied_key, NULL, errp);
>          if (!args) {
>              return NULL;
>          }
> diff --git a/storage-daemon/qemu-storage-daemon.c b/storage-daemon/qemu-storage-daemon.c
> index 1ae1cda481..6f0e0cfb36 100644
> --- a/storage-daemon/qemu-storage-daemon.c
> +++ b/storage-daemon/qemu-storage-daemon.c
> @@ -278,7 +278,7 @@ static void process_options(int argc, char *argv[])
>                  }
>                  qemu_opts_del(opts);
>  
> -                args = keyval_parse(optarg, "qom-type", &error_fatal);
> +                args = keyval_parse(optarg, "qom-type", NULL, &error_fatal);
>                  user_creatable_add_dict(args, true, &error_fatal);
>                  qobject_unref(args);
>                  break;
> diff --git a/tests/test-keyval.c b/tests/test-keyval.c
> index e331a84149..b1433054de 100644
> --- a/tests/test-keyval.c
> +++ b/tests/test-keyval.c
> @@ -27,27 +27,28 @@ static void test_keyval_parse(void)
>      QDict *qdict, *sub_qdict;
>      char long_key[129];
>      char *params;
> +    bool help;
>  
>      /* Nothing */
> -    qdict = keyval_parse("", NULL, &error_abort);
> +    qdict = keyval_parse("", NULL, NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 0);
>      qobject_unref(qdict);
>  
>      /* Empty key (qemu_opts_parse() accepts this) */
> -    qdict = keyval_parse("=val", NULL, &err);
> +    qdict = keyval_parse("=val", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Empty key fragment */
> -    qdict = keyval_parse(".", NULL, &err);
> +    qdict = keyval_parse(".", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
> -    qdict = keyval_parse("key.", NULL, &err);
> +    qdict = keyval_parse("key.", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Invalid non-empty key (qemu_opts_parse() doesn't care) */
> -    qdict = keyval_parse("7up=val", NULL, &err);
> +    qdict = keyval_parse("7up=val", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
> @@ -56,25 +57,25 @@ static void test_keyval_parse(void)
>      long_key[127] = 'z';
>      long_key[128] = 0;
>      params = g_strdup_printf("k.%s=v", long_key);
> -    qdict = keyval_parse(params + 2, NULL, &err);
> +    qdict = keyval_parse(params + 2, NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Overlong key fragment */
> -    qdict = keyval_parse(params, NULL, &err);
> +    qdict = keyval_parse(params, NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>      g_free(params);
>  
>      /* Long key (qemu_opts_parse() accepts and truncates silently) */
>      params = g_strdup_printf("k.%s=v", long_key + 1);
> -    qdict = keyval_parse(params + 2, NULL, &error_abort);
> +    qdict = keyval_parse(params + 2, NULL, NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 1);
>      g_assert_cmpstr(qdict_get_try_str(qdict, long_key + 1), ==, "v");
>      qobject_unref(qdict);
>  
>      /* Long key fragment */
> -    qdict = keyval_parse(params, NULL, &error_abort);
> +    qdict = keyval_parse(params, NULL, NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 1);
>      sub_qdict = qdict_get_qdict(qdict, "k");
>      g_assert(sub_qdict);
> @@ -84,25 +85,25 @@ static void test_keyval_parse(void)
>      g_free(params);
>  
>      /* Crap after valid key */
> -    qdict = keyval_parse("key[0]=val", NULL, &err);
> +    qdict = keyval_parse("key[0]=val", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Multiple keys, last one wins */
> -    qdict = keyval_parse("a=1,b=2,,x,a=3", NULL, &error_abort);
> +    qdict = keyval_parse("a=1,b=2,,x,a=3", NULL, NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 2);
>      g_assert_cmpstr(qdict_get_try_str(qdict, "a"), ==, "3");
>      g_assert_cmpstr(qdict_get_try_str(qdict, "b"), ==, "2,x");
>      qobject_unref(qdict);
>  
>      /* Even when it doesn't in qemu_opts_parse() */
> -    qdict = keyval_parse("id=foo,id=bar", NULL, &error_abort);
> +    qdict = keyval_parse("id=foo,id=bar", NULL, NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 1);
>      g_assert_cmpstr(qdict_get_try_str(qdict, "id"), ==, "bar");
>      qobject_unref(qdict);
>  
>      /* Dotted keys */
> -    qdict = keyval_parse("a.b.c=1,a.b.c=2,d=3", NULL, &error_abort);
> +    qdict = keyval_parse("a.b.c=1,a.b.c=2,d=3", NULL, NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 2);
>      sub_qdict = qdict_get_qdict(qdict, "a");
>      g_assert(sub_qdict);
> @@ -115,48 +116,48 @@ static void test_keyval_parse(void)
>      qobject_unref(qdict);
>  
>      /* Inconsistent dotted keys */
> -    qdict = keyval_parse("a.b=1,a=2", NULL, &err);
> +    qdict = keyval_parse("a.b=1,a=2", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
> -    qdict = keyval_parse("a.b=1,a.b.c=2", NULL, &err);
> +    qdict = keyval_parse("a.b=1,a.b.c=2", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Trailing comma is ignored */
> -    qdict = keyval_parse("x=y,", NULL, &error_abort);
> +    qdict = keyval_parse("x=y,", NULL, NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 1);
>      g_assert_cmpstr(qdict_get_try_str(qdict, "x"), ==, "y");
>      qobject_unref(qdict);
>  
>      /* Except when it isn't */
> -    qdict = keyval_parse(",", NULL, &err);
> +    qdict = keyval_parse(",", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Value containing ,id= not misinterpreted as qemu_opts_parse() does */
> -    qdict = keyval_parse("x=,,id=bar", NULL, &error_abort);
> +    qdict = keyval_parse("x=,,id=bar", NULL, NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 1);
>      g_assert_cmpstr(qdict_get_try_str(qdict, "x"), ==, ",id=bar");
>      qobject_unref(qdict);
>  
>      /* Anti-social ID is left to caller (qemu_opts_parse() rejects it) */
> -    qdict = keyval_parse("id=666", NULL, &error_abort);
> +    qdict = keyval_parse("id=666", NULL, NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 1);
>      g_assert_cmpstr(qdict_get_try_str(qdict, "id"), ==, "666");
>      qobject_unref(qdict);
>  
>      /* Implied value not supported (unlike qemu_opts_parse()) */
> -    qdict = keyval_parse("an,noaus,noaus=", NULL, &err);
> +    qdict = keyval_parse("an,noaus,noaus=", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Implied value, key "no" (qemu_opts_parse(): negated empty key) */
> -    qdict = keyval_parse("no", NULL, &err);
> +    qdict = keyval_parse("no", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Implied key */
> -    qdict = keyval_parse("an,aus=off,noaus=", "implied", &error_abort);
> +    qdict = keyval_parse("an,aus=off,noaus=", "implied", NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 3);
>      g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "an");
>      g_assert_cmpstr(qdict_get_try_str(qdict, "aus"), ==, "off");
> @@ -164,7 +165,7 @@ static void test_keyval_parse(void)
>      qobject_unref(qdict);
>  
>      /* Implied dotted key */
> -    qdict = keyval_parse("val", "eins.zwei", &error_abort);
> +    qdict = keyval_parse("val", "eins.zwei", NULL, &error_abort);
>      g_assert_cmpuint(qdict_size(qdict), ==, 1);
>      sub_qdict = qdict_get_qdict(qdict, "eins");
>      g_assert(sub_qdict);
> @@ -173,19 +174,100 @@ static void test_keyval_parse(void)
>      qobject_unref(qdict);
>  
>      /* Implied key with empty value (qemu_opts_parse() accepts this) */
> -    qdict = keyval_parse(",", "implied", &err);
> +    qdict = keyval_parse(",", "implied", NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Likewise (qemu_opts_parse(): implied key with comma value) */
> -    qdict = keyval_parse(",,,a=1", "implied", &err);
> +    qdict = keyval_parse(",,,a=1", "implied", NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Empty key is not an implied key */
> -    qdict = keyval_parse("=val", "implied", &err);
> +    qdict = keyval_parse("=val", "implied", NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
> +
> +    /* "help" is only a help option if it has no value */
> +    qdict = keyval_parse("help=on", NULL, &help, &error_abort);
> +    g_assert_cmpuint(qdict_size(qdict), ==, 1);
> +    g_assert_cmpstr(qdict_get_try_str(qdict, "help"), ==, "on");
> +    g_assert_false(help);
> +    qobject_unref(qdict);
> +
> +    /* Double comma after "help" in an implied key is not a help option */
> +    qdict = keyval_parse("help,,abc", "implied", &help, &error_abort);
> +    g_assert_cmpuint(qdict_size(qdict), ==, 1);
> +    g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "help,abc");
> +    g_assert_false(help);
> +    qobject_unref(qdict);
> +
> +    /* Without implied key and without value, it's an error */
> +    qdict = keyval_parse("help,,abc", NULL, &help, &err);
> +    error_free_or_abort(&err);
> +    g_assert(!qdict);
> +
> +    /* "help" as the only option */
> +    qdict = keyval_parse("help", NULL, &help, &error_abort);
> +    g_assert_cmpuint(qdict_size(qdict), ==, 0);
> +    g_assert_true(help);

No clearer than g_assert(help) (but it shows off knowledge of GLib).

> +    qobject_unref(qdict);
> +
> +    /* "help" is an error when passing NULL for p_help */
> +    qdict = keyval_parse("help", NULL, NULL, &err);
> +    error_free_or_abort(&err);
> +    g_assert(!qdict);
> +
> +    /* "help" as the first part of the key */
> +    qdict = keyval_parse("help.abc", NULL, &help, &err);
> +    error_free_or_abort(&err);
> +    g_assert(!qdict);
> +
> +    /* "help" as the last part of the key */
> +    qdict = keyval_parse("abc.help", NULL, &help, &err);
> +    error_free_or_abort(&err);
> +    g_assert(!qdict);
> +
> +    /* "help" is not a value for the implied key */
> +    qdict = keyval_parse("help", "implied", &help, &error_abort);
> +    g_assert_cmpuint(qdict_size(qdict), ==, 0);
> +    g_assert_true(help);
> +    qobject_unref(qdict);
> +
> +    /* "help" is not a value for the implied key even with NULL for p_help */
> +    qdict = keyval_parse("help", "implied", NULL, &err);
> +    error_free_or_abort(&err);
> +    g_assert(!qdict);
> +
> +    /* "help.abc" is a value for the implied key */
> +    qdict = keyval_parse("help.abc", "implied", &help, &err);
> +    g_assert_cmpuint(qdict_size(qdict), ==, 1);
> +    g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "help.abc");
> +    g_assert_false(help);

No clearer than g_assert(!help).

> +    qobject_unref(qdict);
> +
> +    /* "abc.help" is a value for the implied key */
> +    qdict = keyval_parse("abc.help", "implied", &help, &err);
> +    g_assert_cmpuint(qdict_size(qdict), ==, 1);
> +    g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "abc.help");
> +    g_assert_false(help);
> +    qobject_unref(qdict);
> +
> +    /* Other keys before and after help are still parsed normally */
> +    qdict = keyval_parse("number=42,help,foo=bar", NULL, &help, &error_abort);
> +    g_assert_cmpuint(qdict_size(qdict), ==, 2);
> +    g_assert_cmpstr(qdict_get_try_str(qdict, "number"), ==, "42");
> +    g_assert_cmpstr(qdict_get_try_str(qdict, "foo"), ==, "bar");
> +    g_assert_true(help);
> +    qobject_unref(qdict);
> +
> +    /* ...even with an implied key */
> +    qdict = keyval_parse("val,help,foo=bar", "implied", &help, &error_abort);
> +    g_assert_cmpuint(qdict_size(qdict), ==, 2);
> +    g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "val");
> +    g_assert_cmpstr(qdict_get_try_str(qdict, "foo"), ==, "bar");
> +    g_assert_true(help);
> +    qobject_unref(qdict);
>  }
>  
>  static void check_list012(QList *qlist)
> @@ -210,26 +292,26 @@ static void test_keyval_parse_list(void)
>      QDict *qdict, *sub_qdict;
>  
>      /* Root can't be a list */
> -    qdict = keyval_parse("0=1", NULL, &err);
> +    qdict = keyval_parse("0=1", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* List elements need not be in order */
> -    qdict = keyval_parse("list.0=null,list.2=zwei,list.1=eins",
> -                         NULL, &error_abort);
> +    qdict = keyval_parse("list.0=null,list.2=zwei,list.1=eins", NULL, NULL,
> +                         &error_abort);
>      g_assert_cmpint(qdict_size(qdict), ==, 1);
>      check_list012(qdict_get_qlist(qdict, "list"));
>      qobject_unref(qdict);
>  
>      /* Multiple indexes, last one wins */
>      qdict = keyval_parse("list.1=goner,list.0=null,list.01=eins,list.2=zwei",
> -                         NULL, &error_abort);
> +                         NULL, NULL, &error_abort);
>      g_assert_cmpint(qdict_size(qdict), ==, 1);
>      check_list012(qdict_get_qlist(qdict, "list"));
>      qobject_unref(qdict);
>  
>      /* List at deeper nesting */
> -    qdict = keyval_parse("a.list.1=eins,a.list.00=null,a.list.2=zwei",
> +    qdict = keyval_parse("a.list.1=eins,a.list.00=null,a.list.2=zwei", NULL,
>                           NULL, &error_abort);
>      g_assert_cmpint(qdict_size(qdict), ==, 1);
>      sub_qdict = qdict_get_qdict(qdict, "a");
> @@ -238,18 +320,19 @@ static void test_keyval_parse_list(void)
>      qobject_unref(qdict);
>  
>      /* Inconsistent dotted keys: both list and dictionary */
> -    qdict = keyval_parse("a.b.c=1,a.b.0=2", NULL, &err);
> +    qdict = keyval_parse("a.b.c=1,a.b.0=2", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
> -    qdict = keyval_parse("a.0.c=1,a.b.c=2", NULL, &err);
> +    qdict = keyval_parse("a.0.c=1,a.b.c=2", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  
>      /* Missing list indexes */
> -    qdict = keyval_parse("list.1=lonely", NULL, &err);
> +    qdict = keyval_parse("list.1=lonely", NULL, NULL, &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
> -    qdict = keyval_parse("list.0=null,list.2=eins,list.02=zwei", NULL, &err);
> +    qdict = keyval_parse("list.0=null,list.2=eins,list.02=zwei", NULL, NULL,
> +                         &err);
>      error_free_or_abort(&err);
>      g_assert(!qdict);
>  }
> @@ -261,7 +344,7 @@ static void test_keyval_visit_bool(void)
>      QDict *qdict;
>      bool b;
>  
> -    qdict = keyval_parse("bool1=on,bool2=off", NULL, &error_abort);
> +    qdict = keyval_parse("bool1=on,bool2=off", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -273,7 +356,7 @@ static void test_keyval_visit_bool(void)
>      visit_end_struct(v, NULL);
>      visit_free(v);
>  
> -    qdict = keyval_parse("bool1=offer", NULL, &error_abort);
> +    qdict = keyval_parse("bool1=offer", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -291,7 +374,7 @@ static void test_keyval_visit_number(void)
>      uint64_t u;
>  
>      /* Lower limit zero */
> -    qdict = keyval_parse("number1=0", NULL, &error_abort);
> +    qdict = keyval_parse("number1=0", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -302,7 +385,7 @@ static void test_keyval_visit_number(void)
>      visit_free(v);
>  
>      /* Upper limit 2^64-1 */
> -    qdict = keyval_parse("number1=18446744073709551615,number2=-1",
> +    qdict = keyval_parse("number1=18446744073709551615,number2=-1", NULL,
>                           NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
> @@ -316,8 +399,8 @@ static void test_keyval_visit_number(void)
>      visit_free(v);
>  
>      /* Above upper limit */
> -    qdict = keyval_parse("number1=18446744073709551616",
> -                         NULL, &error_abort);
> +    qdict = keyval_parse("number1=18446744073709551616", NULL, NULL,
> +                         &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -327,8 +410,8 @@ static void test_keyval_visit_number(void)
>      visit_free(v);
>  
>      /* Below lower limit */
> -    qdict = keyval_parse("number1=-18446744073709551616",
> -                         NULL, &error_abort);
> +    qdict = keyval_parse("number1=-18446744073709551616", NULL, NULL,
> +                         &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -338,8 +421,7 @@ static void test_keyval_visit_number(void)
>      visit_free(v);
>  
>      /* Hex and octal */
> -    qdict = keyval_parse("number1=0x2a,number2=052",
> -                         NULL, &error_abort);
> +    qdict = keyval_parse("number1=0x2a,number2=052", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -352,8 +434,7 @@ static void test_keyval_visit_number(void)
>      visit_free(v);
>  
>      /* Trailing crap */
> -    qdict = keyval_parse("number1=3.14,number2=08",
> -                         NULL, &error_abort);
> +    qdict = keyval_parse("number1=3.14,number2=08", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -373,7 +454,7 @@ static void test_keyval_visit_size(void)
>      uint64_t sz;
>  
>      /* Lower limit zero */
> -    qdict = keyval_parse("sz1=0", NULL, &error_abort);
> +    qdict = keyval_parse("sz1=0", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -389,7 +470,7 @@ static void test_keyval_visit_size(void)
>      qdict = keyval_parse("sz1=9007199254740991,"
>                           "sz2=9007199254740992,"
>                           "sz3=9007199254740993",
> -                         NULL, &error_abort);
> +                         NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -406,7 +487,7 @@ static void test_keyval_visit_size(void)
>      /* Close to signed upper limit 0x7ffffffffffffc00 (53 msbs set) */
>      qdict = keyval_parse("sz1=9223372036854774784," /* 7ffffffffffffc00 */
>                           "sz2=9223372036854775295", /* 7ffffffffffffdff */
> -                         NULL, &error_abort);
> +                         NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -421,7 +502,7 @@ static void test_keyval_visit_size(void)
>      /* Close to actual upper limit 0xfffffffffffff800 (53 msbs set) */
>      qdict = keyval_parse("sz1=18446744073709549568," /* fffffffffffff800 */
>                           "sz2=18446744073709550591", /* fffffffffffffbff */
> -                         NULL, &error_abort);
> +                         NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -436,7 +517,7 @@ static void test_keyval_visit_size(void)
>      /* Beyond limits */
>      qdict = keyval_parse("sz1=-1,"
>                           "sz2=18446744073709550592", /* fffffffffffffc00 */
> -                         NULL, &error_abort);
> +                         NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -449,7 +530,7 @@ static void test_keyval_visit_size(void)
>  
>      /* Suffixes */
>      qdict = keyval_parse("sz1=8b,sz2=1.5k,sz3=2M,sz4=0.1G,sz5=16777215T",
> -                         NULL, &error_abort);
> +                         NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -468,7 +549,7 @@ static void test_keyval_visit_size(void)
>      visit_free(v);
>  
>      /* Beyond limit with suffix */
> -    qdict = keyval_parse("sz1=16777216T", NULL, &error_abort);
> +    qdict = keyval_parse("sz1=16777216T", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -478,7 +559,7 @@ static void test_keyval_visit_size(void)
>      visit_free(v);
>  
>      /* Trailing crap */
> -    qdict = keyval_parse("sz1=0Z,sz2=16Gi", NULL, &error_abort);
> +    qdict = keyval_parse("sz1=0Z,sz2=16Gi", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -497,7 +578,7 @@ static void test_keyval_visit_dict(void)
>      QDict *qdict;
>      int64_t i;
>  
> -    qdict = keyval_parse("a.b.c=1,a.b.c=2,d=3", NULL, &error_abort);
> +    qdict = keyval_parse("a.b.c=1,a.b.c=2,d=3", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -515,7 +596,7 @@ static void test_keyval_visit_dict(void)
>      visit_end_struct(v, NULL);
>      visit_free(v);
>  
> -    qdict = keyval_parse("a.b=", NULL, &error_abort);
> +    qdict = keyval_parse("a.b=", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -537,7 +618,7 @@ static void test_keyval_visit_list(void)
>      QDict *qdict;
>      char *s;
>  
> -    qdict = keyval_parse("a.0=,a.1=I,a.2.0=II", NULL, &error_abort);
> +    qdict = keyval_parse("a.0=,a.1=I,a.2.0=II", NULL, NULL, &error_abort);
>      /* TODO empty list */
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
> @@ -561,7 +642,7 @@ static void test_keyval_visit_list(void)
>      visit_end_struct(v, NULL);
>      visit_free(v);
>  
> -    qdict = keyval_parse("a.0=,b.0.0=head", NULL, &error_abort);
> +    qdict = keyval_parse("a.0=,b.0.0=head", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -590,7 +671,7 @@ static void test_keyval_visit_optional(void)
>      bool present;
>      int64_t i;
>  
> -    qdict = keyval_parse("a.b=1", NULL, &error_abort);
> +    qdict = keyval_parse("a.b=1", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -626,7 +707,7 @@ static void test_keyval_visit_alternate(void)
>       * the string variant if there is one, else an error.
>       * TODO make it work for unambiguous cases like AltEnumBool below
>       */
> -    qdict = keyval_parse("a=1,b=2,c=on", NULL, &error_abort);
> +    qdict = keyval_parse("a=1,b=2,c=on", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> @@ -650,7 +731,7 @@ static void test_keyval_visit_any(void)
>      QList *qlist;
>      QString *qstr;
>  
> -    qdict = keyval_parse("a.0=null,a.1=1", NULL, &error_abort);
> +    qdict = keyval_parse("a.0=null,a.1=1", NULL, NULL, &error_abort);
>      v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
>      qobject_unref(qdict);
>      visit_start_struct(v, NULL, NULL, 0, &error_abort);
> diff --git a/util/keyval.c b/util/keyval.c
> index 13def4af54..5929c9dba2 100644
> --- a/util/keyval.c
> +++ b/util/keyval.c
> @@ -14,7 +14,7 @@
>   * KEY=VALUE,... syntax:
>   *
>   *   key-vals     = [ key-val { ',' key-val } [ ',' ] ]
> - *   key-val      = key '=' val
> + *   key-val      = 'help' | key '=' val
>   *   key          = key-fragment { '.' key-fragment }
>   *   key-fragment = / [^=,.]* /
>   *   val          = { / [^,]* / | ',,' }
> @@ -73,10 +73,14 @@
>   *
>   * Additional syntax for use with an implied key:
>   *
> - *   key-vals-ik  = val-no-key [ ',' key-vals ]
> + *   key-vals-ik  = 'help' | val-no-key [ ',' key-vals ]
>   *   val-no-key   = / [^=,]* /
>   *
>   * where no-key is syntactic sugar for implied-key=val-no-key.
> + *
> + * 'help' for key-val or key-vals-ik is not interpreted as part of the
> + * JSON object, but instead requests help, which is a separate boolean
> + * output of the parser.
>   */
>  
>  #include "qemu/osdep.h"
> @@ -85,6 +89,7 @@
>  #include "qapi/qmp/qlist.h"
>  #include "qapi/qmp/qstring.h"
>  #include "qemu/cutils.h"
> +#include "qemu/help_option.h"
>  #include "qemu/option.h"
>  
>  /*
> @@ -166,7 +171,7 @@ static QObject *keyval_parse_put(QDict *cur,
>   * On failure, return NULL.
>   */

Contract needs an update.

>  static const char *keyval_parse_one(QDict *qdict, const char *params,
> -                                    const char *implied_key,
> +                                    const char *implied_key, bool *help,
>                                      Error **errp)
>  {
>      const char *key, *key_end, *s, *end;
> @@ -238,13 +243,20 @@ static const char *keyval_parse_one(QDict *qdict, const char *params,
>      if (key == implied_key) {
>          assert(!*s);
>          s = params;
> +    } else if (*s == '=') {
> +        s++;
>      } else {
> -        if (*s != '=') {
> +        if (is_help_option_n(key, s - key)) {
> +            *help = true;
> +            if (*s) {
> +                s++;
> +            }
> +            return s;
> +        } else {
>              error_setg(errp, "Expected '=' after parameter '%.*s'",
>                         (int)(s - key), key);
>              return NULL;
>          }
> -        s++;
>      }
>  
>      val = qstring_new();
> @@ -260,6 +272,15 @@ static const char *keyval_parse_one(QDict *qdict, const char *params,
>          qstring_append_chr(val, *s++);
>      }
>  
> +    if (key == implied_key) {
> +        const char *str_val = qstring_get_str(val);
> +        if (is_help_option(str_val)) {
> +            *help = true;
> +            qobject_unref(val);
> +            return s;
> +        }
> +    }
> +
>      if (!keyval_parse_put(cur, key_in_cur, val, key, key_end, errp)) {
>          return NULL;
>      }

The patch is minimal, and I believe works except for one corner case
(search for Bug: below).  But it makes the code even more subtle than it
already is.

Code before the patch:

       key = params;
       len = strcspn(params, "=,");

Both key and val-no-key are / [^=,]* /.  The former is followed by '=',
the latter is not.  This lets us distinguish the two.

       if (implied_key && len && key[len] != '=') {
           /* Desugar implied key */
           key = implied_key;
           len = strlen(implied_key);
       }
       key_end = key + len;

@key, @key_end now bracket what must be the key.

       [parse from @key to @key_end...]

       if (key == implied_key) {
           assert(!*s);
           s = params;

We're parsing val-no-key, and @s points to it.

       } else {
           if (*s != '=') {
               error_setg(errp, "Expected '=' after parameter '%.*s'",
                          (int)(s - key), key);
               return NULL;
           }
           s++;

We're parsing key '=' val, and @s points to val.

       }

       [parse from @s to unescaped ',' or to '\0'... into @val]

       if (!keyval_parse_put(cur, key_in_cur, val, key, key_end, errp)) {
           return NULL;
       }
       return s;

The patch adds parsing of help.  How it is parsed depends on
@implied_key.

Without implied_key:

       key = params;
       len = strcspn(params, "=,");
       if (implied_key && len && key[len] != '=') {
           [not reached...]
       }
       key_end = key + len;

@key, @key_end now bracket what must be the key or help.

       [parse from @key to @key_end...]

Bug: parsing help as if it was a key works for 'help' (which parses okay
as key), but not for '?' (which doesn't).  Reproducer:

    $ upstream-qemu-storage-daemon --nbd-server help
    upstream-qemu-storage-daemon: Help is not available for this option
    $ upstream-qemu-storage-daemon --nbd-server '?'
    upstream-qemu-storage-daemon: Invalid parameter '?'

       if (key == implied_key) {
           [not reached...]
       } else if (*s == '=') {
           s++;

We're parsing key '=' val, and @s points to val.

       } else {

We're parsing help (if we survived parsing help as key).

           if (is_help_option_n(key, s - key)) {
               *help = true;
               if (*s) {
                   s++;
               }
               return s;
           } else {
               error_setg(errp, "Expected '=' after parameter '%.*s'",
                          (int)(s - key), key);
               return NULL;
           }
       }

       [parse from @s to unescaped ',' or to '\0'... into @val]

       if (key == implied_key) {
           [not reached...]
       }

       if (!keyval_parse_put(cur, key_in_cur, val, key, key_end, errp)) {
           return NULL;
       }
       return s;

With implied_key:

       key = params;
       len = strcspn(params, "=,");
       if (implied_key && len && key[len] != '=') {
           /* Desugar implied key */

help takes this branch, too.

           key = implied_key;
           len = strlen(implied_key);
       }
       key_end = key + len;

@key, @key_end now bracket what must be the key.  If we're looking at
help, it's the implied key, and it'll be ignored, but ...

       [parse from @key to @key_end...]

... we parse it all the same.  No harm in doing that.

       if (key == implied_key) {
           assert(!*s);
           s = params;

We're parsing val-no-key | help, and @s points to it.

       } else if (*s == '=') {
           s++;

We're parsing key '=' val, and @s points to val.

       } else {

Nothing left to parse; we're going to error no matter what.

           if (is_help_option_n(key, s - key)) {
               [not reached...]
           } else {
               error_setg(errp, "Expected '=' after parameter '%.*s'",
                          (int)(s - key), key);
               return NULL;
           }
       }

       [parse from @s to unescaped ',' or to '\0'... into @val]

       if (key == implied_key) {

We parsed val-no-key | help.

           const char *str_val = qstring_get_str(val);
           if (is_help_option(str_val)) {

It's help.

               *help = true;
               qobject_unref(val);
               return s;
           }
       }

       if (!keyval_parse_put(cur, key_in_cur, val, key, key_end, errp)) {
           return NULL;
       }
       return s;

Your v1 was much simpler.  Eric asked "what if the user typed "help,,"
to get "help," as the value of the implied key?"  Good question, but the
correct answer is that the value of an implied key cannot contain ',',
which means v1 wasn't actually flawed.

I'd like to tighten up grammar, code, and tests in a patch that goes
first.  Then I adjust this one, and post the whole as v4.  Okay?

[...]
diff mbox series

Patch

diff --git a/include/qemu/help_option.h b/include/qemu/help_option.h
index 328d2a89fd..952d628b9d 100644
--- a/include/qemu/help_option.h
+++ b/include/qemu/help_option.h
@@ -19,4 +19,9 @@  static inline bool is_help_option(const char *s)
     return !strcmp(s, "?") || !strcmp(s, "help");
 }
 
+static inline bool is_help_option_n(const char *s, size_t n)
+{
+    return !strncmp(s, "?", n) || !strncmp(s, "help", n);
+}
+
 #endif
diff --git a/include/qemu/option.h b/include/qemu/option.h
index 05e8a15c73..ac69352e0e 100644
--- a/include/qemu/option.h
+++ b/include/qemu/option.h
@@ -149,6 +149,6 @@  void qemu_opts_free(QemuOptsList *list);
 QemuOptsList *qemu_opts_append(QemuOptsList *dst, QemuOptsList *list);
 
 QDict *keyval_parse(const char *params, const char *implied_key,
-                    Error **errp);
+                    bool *help, Error **errp);
 
 #endif
diff --git a/qapi/qobject-input-visitor.c b/qapi/qobject-input-visitor.c
index f918a05e5f..7b184b50a7 100644
--- a/qapi/qobject-input-visitor.c
+++ b/qapi/qobject-input-visitor.c
@@ -757,7 +757,7 @@  Visitor *qobject_input_visitor_new_str(const char *str,
         assert(args);
         v = qobject_input_visitor_new(QOBJECT(args));
     } else {
-        args = keyval_parse(str, implied_key, errp);
+        args = keyval_parse(str, implied_key, NULL, errp);
         if (!args) {
             return NULL;
         }
diff --git a/storage-daemon/qemu-storage-daemon.c b/storage-daemon/qemu-storage-daemon.c
index 1ae1cda481..6f0e0cfb36 100644
--- a/storage-daemon/qemu-storage-daemon.c
+++ b/storage-daemon/qemu-storage-daemon.c
@@ -278,7 +278,7 @@  static void process_options(int argc, char *argv[])
                 }
                 qemu_opts_del(opts);
 
-                args = keyval_parse(optarg, "qom-type", &error_fatal);
+                args = keyval_parse(optarg, "qom-type", NULL, &error_fatal);
                 user_creatable_add_dict(args, true, &error_fatal);
                 qobject_unref(args);
                 break;
diff --git a/tests/test-keyval.c b/tests/test-keyval.c
index e331a84149..b1433054de 100644
--- a/tests/test-keyval.c
+++ b/tests/test-keyval.c
@@ -27,27 +27,28 @@  static void test_keyval_parse(void)
     QDict *qdict, *sub_qdict;
     char long_key[129];
     char *params;
+    bool help;
 
     /* Nothing */
-    qdict = keyval_parse("", NULL, &error_abort);
+    qdict = keyval_parse("", NULL, NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 0);
     qobject_unref(qdict);
 
     /* Empty key (qemu_opts_parse() accepts this) */
-    qdict = keyval_parse("=val", NULL, &err);
+    qdict = keyval_parse("=val", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Empty key fragment */
-    qdict = keyval_parse(".", NULL, &err);
+    qdict = keyval_parse(".", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
-    qdict = keyval_parse("key.", NULL, &err);
+    qdict = keyval_parse("key.", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Invalid non-empty key (qemu_opts_parse() doesn't care) */
-    qdict = keyval_parse("7up=val", NULL, &err);
+    qdict = keyval_parse("7up=val", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
@@ -56,25 +57,25 @@  static void test_keyval_parse(void)
     long_key[127] = 'z';
     long_key[128] = 0;
     params = g_strdup_printf("k.%s=v", long_key);
-    qdict = keyval_parse(params + 2, NULL, &err);
+    qdict = keyval_parse(params + 2, NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Overlong key fragment */
-    qdict = keyval_parse(params, NULL, &err);
+    qdict = keyval_parse(params, NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
     g_free(params);
 
     /* Long key (qemu_opts_parse() accepts and truncates silently) */
     params = g_strdup_printf("k.%s=v", long_key + 1);
-    qdict = keyval_parse(params + 2, NULL, &error_abort);
+    qdict = keyval_parse(params + 2, NULL, NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 1);
     g_assert_cmpstr(qdict_get_try_str(qdict, long_key + 1), ==, "v");
     qobject_unref(qdict);
 
     /* Long key fragment */
-    qdict = keyval_parse(params, NULL, &error_abort);
+    qdict = keyval_parse(params, NULL, NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 1);
     sub_qdict = qdict_get_qdict(qdict, "k");
     g_assert(sub_qdict);
@@ -84,25 +85,25 @@  static void test_keyval_parse(void)
     g_free(params);
 
     /* Crap after valid key */
-    qdict = keyval_parse("key[0]=val", NULL, &err);
+    qdict = keyval_parse("key[0]=val", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Multiple keys, last one wins */
-    qdict = keyval_parse("a=1,b=2,,x,a=3", NULL, &error_abort);
+    qdict = keyval_parse("a=1,b=2,,x,a=3", NULL, NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 2);
     g_assert_cmpstr(qdict_get_try_str(qdict, "a"), ==, "3");
     g_assert_cmpstr(qdict_get_try_str(qdict, "b"), ==, "2,x");
     qobject_unref(qdict);
 
     /* Even when it doesn't in qemu_opts_parse() */
-    qdict = keyval_parse("id=foo,id=bar", NULL, &error_abort);
+    qdict = keyval_parse("id=foo,id=bar", NULL, NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 1);
     g_assert_cmpstr(qdict_get_try_str(qdict, "id"), ==, "bar");
     qobject_unref(qdict);
 
     /* Dotted keys */
-    qdict = keyval_parse("a.b.c=1,a.b.c=2,d=3", NULL, &error_abort);
+    qdict = keyval_parse("a.b.c=1,a.b.c=2,d=3", NULL, NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 2);
     sub_qdict = qdict_get_qdict(qdict, "a");
     g_assert(sub_qdict);
@@ -115,48 +116,48 @@  static void test_keyval_parse(void)
     qobject_unref(qdict);
 
     /* Inconsistent dotted keys */
-    qdict = keyval_parse("a.b=1,a=2", NULL, &err);
+    qdict = keyval_parse("a.b=1,a=2", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
-    qdict = keyval_parse("a.b=1,a.b.c=2", NULL, &err);
+    qdict = keyval_parse("a.b=1,a.b.c=2", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Trailing comma is ignored */
-    qdict = keyval_parse("x=y,", NULL, &error_abort);
+    qdict = keyval_parse("x=y,", NULL, NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 1);
     g_assert_cmpstr(qdict_get_try_str(qdict, "x"), ==, "y");
     qobject_unref(qdict);
 
     /* Except when it isn't */
-    qdict = keyval_parse(",", NULL, &err);
+    qdict = keyval_parse(",", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Value containing ,id= not misinterpreted as qemu_opts_parse() does */
-    qdict = keyval_parse("x=,,id=bar", NULL, &error_abort);
+    qdict = keyval_parse("x=,,id=bar", NULL, NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 1);
     g_assert_cmpstr(qdict_get_try_str(qdict, "x"), ==, ",id=bar");
     qobject_unref(qdict);
 
     /* Anti-social ID is left to caller (qemu_opts_parse() rejects it) */
-    qdict = keyval_parse("id=666", NULL, &error_abort);
+    qdict = keyval_parse("id=666", NULL, NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 1);
     g_assert_cmpstr(qdict_get_try_str(qdict, "id"), ==, "666");
     qobject_unref(qdict);
 
     /* Implied value not supported (unlike qemu_opts_parse()) */
-    qdict = keyval_parse("an,noaus,noaus=", NULL, &err);
+    qdict = keyval_parse("an,noaus,noaus=", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Implied value, key "no" (qemu_opts_parse(): negated empty key) */
-    qdict = keyval_parse("no", NULL, &err);
+    qdict = keyval_parse("no", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Implied key */
-    qdict = keyval_parse("an,aus=off,noaus=", "implied", &error_abort);
+    qdict = keyval_parse("an,aus=off,noaus=", "implied", NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 3);
     g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "an");
     g_assert_cmpstr(qdict_get_try_str(qdict, "aus"), ==, "off");
@@ -164,7 +165,7 @@  static void test_keyval_parse(void)
     qobject_unref(qdict);
 
     /* Implied dotted key */
-    qdict = keyval_parse("val", "eins.zwei", &error_abort);
+    qdict = keyval_parse("val", "eins.zwei", NULL, &error_abort);
     g_assert_cmpuint(qdict_size(qdict), ==, 1);
     sub_qdict = qdict_get_qdict(qdict, "eins");
     g_assert(sub_qdict);
@@ -173,19 +174,100 @@  static void test_keyval_parse(void)
     qobject_unref(qdict);
 
     /* Implied key with empty value (qemu_opts_parse() accepts this) */
-    qdict = keyval_parse(",", "implied", &err);
+    qdict = keyval_parse(",", "implied", NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Likewise (qemu_opts_parse(): implied key with comma value) */
-    qdict = keyval_parse(",,,a=1", "implied", &err);
+    qdict = keyval_parse(",,,a=1", "implied", NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Empty key is not an implied key */
-    qdict = keyval_parse("=val", "implied", &err);
+    qdict = keyval_parse("=val", "implied", NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
+
+    /* "help" is only a help option if it has no value */
+    qdict = keyval_parse("help=on", NULL, &help, &error_abort);
+    g_assert_cmpuint(qdict_size(qdict), ==, 1);
+    g_assert_cmpstr(qdict_get_try_str(qdict, "help"), ==, "on");
+    g_assert_false(help);
+    qobject_unref(qdict);
+
+    /* Double comma after "help" in an implied key is not a help option */
+    qdict = keyval_parse("help,,abc", "implied", &help, &error_abort);
+    g_assert_cmpuint(qdict_size(qdict), ==, 1);
+    g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "help,abc");
+    g_assert_false(help);
+    qobject_unref(qdict);
+
+    /* Without implied key and without value, it's an error */
+    qdict = keyval_parse("help,,abc", NULL, &help, &err);
+    error_free_or_abort(&err);
+    g_assert(!qdict);
+
+    /* "help" as the only option */
+    qdict = keyval_parse("help", NULL, &help, &error_abort);
+    g_assert_cmpuint(qdict_size(qdict), ==, 0);
+    g_assert_true(help);
+    qobject_unref(qdict);
+
+    /* "help" is an error when passing NULL for p_help */
+    qdict = keyval_parse("help", NULL, NULL, &err);
+    error_free_or_abort(&err);
+    g_assert(!qdict);
+
+    /* "help" as the first part of the key */
+    qdict = keyval_parse("help.abc", NULL, &help, &err);
+    error_free_or_abort(&err);
+    g_assert(!qdict);
+
+    /* "help" as the last part of the key */
+    qdict = keyval_parse("abc.help", NULL, &help, &err);
+    error_free_or_abort(&err);
+    g_assert(!qdict);
+
+    /* "help" is not a value for the implied key */
+    qdict = keyval_parse("help", "implied", &help, &error_abort);
+    g_assert_cmpuint(qdict_size(qdict), ==, 0);
+    g_assert_true(help);
+    qobject_unref(qdict);
+
+    /* "help" is not a value for the implied key even with NULL for p_help */
+    qdict = keyval_parse("help", "implied", NULL, &err);
+    error_free_or_abort(&err);
+    g_assert(!qdict);
+
+    /* "help.abc" is a value for the implied key */
+    qdict = keyval_parse("help.abc", "implied", &help, &err);
+    g_assert_cmpuint(qdict_size(qdict), ==, 1);
+    g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "help.abc");
+    g_assert_false(help);
+    qobject_unref(qdict);
+
+    /* "abc.help" is a value for the implied key */
+    qdict = keyval_parse("abc.help", "implied", &help, &err);
+    g_assert_cmpuint(qdict_size(qdict), ==, 1);
+    g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "abc.help");
+    g_assert_false(help);
+    qobject_unref(qdict);
+
+    /* Other keys before and after help are still parsed normally */
+    qdict = keyval_parse("number=42,help,foo=bar", NULL, &help, &error_abort);
+    g_assert_cmpuint(qdict_size(qdict), ==, 2);
+    g_assert_cmpstr(qdict_get_try_str(qdict, "number"), ==, "42");
+    g_assert_cmpstr(qdict_get_try_str(qdict, "foo"), ==, "bar");
+    g_assert_true(help);
+    qobject_unref(qdict);
+
+    /* ...even with an implied key */
+    qdict = keyval_parse("val,help,foo=bar", "implied", &help, &error_abort);
+    g_assert_cmpuint(qdict_size(qdict), ==, 2);
+    g_assert_cmpstr(qdict_get_try_str(qdict, "implied"), ==, "val");
+    g_assert_cmpstr(qdict_get_try_str(qdict, "foo"), ==, "bar");
+    g_assert_true(help);
+    qobject_unref(qdict);
 }
 
 static void check_list012(QList *qlist)
@@ -210,26 +292,26 @@  static void test_keyval_parse_list(void)
     QDict *qdict, *sub_qdict;
 
     /* Root can't be a list */
-    qdict = keyval_parse("0=1", NULL, &err);
+    qdict = keyval_parse("0=1", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* List elements need not be in order */
-    qdict = keyval_parse("list.0=null,list.2=zwei,list.1=eins",
-                         NULL, &error_abort);
+    qdict = keyval_parse("list.0=null,list.2=zwei,list.1=eins", NULL, NULL,
+                         &error_abort);
     g_assert_cmpint(qdict_size(qdict), ==, 1);
     check_list012(qdict_get_qlist(qdict, "list"));
     qobject_unref(qdict);
 
     /* Multiple indexes, last one wins */
     qdict = keyval_parse("list.1=goner,list.0=null,list.01=eins,list.2=zwei",
-                         NULL, &error_abort);
+                         NULL, NULL, &error_abort);
     g_assert_cmpint(qdict_size(qdict), ==, 1);
     check_list012(qdict_get_qlist(qdict, "list"));
     qobject_unref(qdict);
 
     /* List at deeper nesting */
-    qdict = keyval_parse("a.list.1=eins,a.list.00=null,a.list.2=zwei",
+    qdict = keyval_parse("a.list.1=eins,a.list.00=null,a.list.2=zwei", NULL,
                          NULL, &error_abort);
     g_assert_cmpint(qdict_size(qdict), ==, 1);
     sub_qdict = qdict_get_qdict(qdict, "a");
@@ -238,18 +320,19 @@  static void test_keyval_parse_list(void)
     qobject_unref(qdict);
 
     /* Inconsistent dotted keys: both list and dictionary */
-    qdict = keyval_parse("a.b.c=1,a.b.0=2", NULL, &err);
+    qdict = keyval_parse("a.b.c=1,a.b.0=2", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
-    qdict = keyval_parse("a.0.c=1,a.b.c=2", NULL, &err);
+    qdict = keyval_parse("a.0.c=1,a.b.c=2", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 
     /* Missing list indexes */
-    qdict = keyval_parse("list.1=lonely", NULL, &err);
+    qdict = keyval_parse("list.1=lonely", NULL, NULL, &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
-    qdict = keyval_parse("list.0=null,list.2=eins,list.02=zwei", NULL, &err);
+    qdict = keyval_parse("list.0=null,list.2=eins,list.02=zwei", NULL, NULL,
+                         &err);
     error_free_or_abort(&err);
     g_assert(!qdict);
 }
@@ -261,7 +344,7 @@  static void test_keyval_visit_bool(void)
     QDict *qdict;
     bool b;
 
-    qdict = keyval_parse("bool1=on,bool2=off", NULL, &error_abort);
+    qdict = keyval_parse("bool1=on,bool2=off", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -273,7 +356,7 @@  static void test_keyval_visit_bool(void)
     visit_end_struct(v, NULL);
     visit_free(v);
 
-    qdict = keyval_parse("bool1=offer", NULL, &error_abort);
+    qdict = keyval_parse("bool1=offer", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -291,7 +374,7 @@  static void test_keyval_visit_number(void)
     uint64_t u;
 
     /* Lower limit zero */
-    qdict = keyval_parse("number1=0", NULL, &error_abort);
+    qdict = keyval_parse("number1=0", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -302,7 +385,7 @@  static void test_keyval_visit_number(void)
     visit_free(v);
 
     /* Upper limit 2^64-1 */
-    qdict = keyval_parse("number1=18446744073709551615,number2=-1",
+    qdict = keyval_parse("number1=18446744073709551615,number2=-1", NULL,
                          NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
@@ -316,8 +399,8 @@  static void test_keyval_visit_number(void)
     visit_free(v);
 
     /* Above upper limit */
-    qdict = keyval_parse("number1=18446744073709551616",
-                         NULL, &error_abort);
+    qdict = keyval_parse("number1=18446744073709551616", NULL, NULL,
+                         &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -327,8 +410,8 @@  static void test_keyval_visit_number(void)
     visit_free(v);
 
     /* Below lower limit */
-    qdict = keyval_parse("number1=-18446744073709551616",
-                         NULL, &error_abort);
+    qdict = keyval_parse("number1=-18446744073709551616", NULL, NULL,
+                         &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -338,8 +421,7 @@  static void test_keyval_visit_number(void)
     visit_free(v);
 
     /* Hex and octal */
-    qdict = keyval_parse("number1=0x2a,number2=052",
-                         NULL, &error_abort);
+    qdict = keyval_parse("number1=0x2a,number2=052", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -352,8 +434,7 @@  static void test_keyval_visit_number(void)
     visit_free(v);
 
     /* Trailing crap */
-    qdict = keyval_parse("number1=3.14,number2=08",
-                         NULL, &error_abort);
+    qdict = keyval_parse("number1=3.14,number2=08", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -373,7 +454,7 @@  static void test_keyval_visit_size(void)
     uint64_t sz;
 
     /* Lower limit zero */
-    qdict = keyval_parse("sz1=0", NULL, &error_abort);
+    qdict = keyval_parse("sz1=0", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -389,7 +470,7 @@  static void test_keyval_visit_size(void)
     qdict = keyval_parse("sz1=9007199254740991,"
                          "sz2=9007199254740992,"
                          "sz3=9007199254740993",
-                         NULL, &error_abort);
+                         NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -406,7 +487,7 @@  static void test_keyval_visit_size(void)
     /* Close to signed upper limit 0x7ffffffffffffc00 (53 msbs set) */
     qdict = keyval_parse("sz1=9223372036854774784," /* 7ffffffffffffc00 */
                          "sz2=9223372036854775295", /* 7ffffffffffffdff */
-                         NULL, &error_abort);
+                         NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -421,7 +502,7 @@  static void test_keyval_visit_size(void)
     /* Close to actual upper limit 0xfffffffffffff800 (53 msbs set) */
     qdict = keyval_parse("sz1=18446744073709549568," /* fffffffffffff800 */
                          "sz2=18446744073709550591", /* fffffffffffffbff */
-                         NULL, &error_abort);
+                         NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -436,7 +517,7 @@  static void test_keyval_visit_size(void)
     /* Beyond limits */
     qdict = keyval_parse("sz1=-1,"
                          "sz2=18446744073709550592", /* fffffffffffffc00 */
-                         NULL, &error_abort);
+                         NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -449,7 +530,7 @@  static void test_keyval_visit_size(void)
 
     /* Suffixes */
     qdict = keyval_parse("sz1=8b,sz2=1.5k,sz3=2M,sz4=0.1G,sz5=16777215T",
-                         NULL, &error_abort);
+                         NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -468,7 +549,7 @@  static void test_keyval_visit_size(void)
     visit_free(v);
 
     /* Beyond limit with suffix */
-    qdict = keyval_parse("sz1=16777216T", NULL, &error_abort);
+    qdict = keyval_parse("sz1=16777216T", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -478,7 +559,7 @@  static void test_keyval_visit_size(void)
     visit_free(v);
 
     /* Trailing crap */
-    qdict = keyval_parse("sz1=0Z,sz2=16Gi", NULL, &error_abort);
+    qdict = keyval_parse("sz1=0Z,sz2=16Gi", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -497,7 +578,7 @@  static void test_keyval_visit_dict(void)
     QDict *qdict;
     int64_t i;
 
-    qdict = keyval_parse("a.b.c=1,a.b.c=2,d=3", NULL, &error_abort);
+    qdict = keyval_parse("a.b.c=1,a.b.c=2,d=3", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -515,7 +596,7 @@  static void test_keyval_visit_dict(void)
     visit_end_struct(v, NULL);
     visit_free(v);
 
-    qdict = keyval_parse("a.b=", NULL, &error_abort);
+    qdict = keyval_parse("a.b=", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -537,7 +618,7 @@  static void test_keyval_visit_list(void)
     QDict *qdict;
     char *s;
 
-    qdict = keyval_parse("a.0=,a.1=I,a.2.0=II", NULL, &error_abort);
+    qdict = keyval_parse("a.0=,a.1=I,a.2.0=II", NULL, NULL, &error_abort);
     /* TODO empty list */
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
@@ -561,7 +642,7 @@  static void test_keyval_visit_list(void)
     visit_end_struct(v, NULL);
     visit_free(v);
 
-    qdict = keyval_parse("a.0=,b.0.0=head", NULL, &error_abort);
+    qdict = keyval_parse("a.0=,b.0.0=head", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -590,7 +671,7 @@  static void test_keyval_visit_optional(void)
     bool present;
     int64_t i;
 
-    qdict = keyval_parse("a.b=1", NULL, &error_abort);
+    qdict = keyval_parse("a.b=1", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -626,7 +707,7 @@  static void test_keyval_visit_alternate(void)
      * the string variant if there is one, else an error.
      * TODO make it work for unambiguous cases like AltEnumBool below
      */
-    qdict = keyval_parse("a=1,b=2,c=on", NULL, &error_abort);
+    qdict = keyval_parse("a=1,b=2,c=on", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
@@ -650,7 +731,7 @@  static void test_keyval_visit_any(void)
     QList *qlist;
     QString *qstr;
 
-    qdict = keyval_parse("a.0=null,a.1=1", NULL, &error_abort);
+    qdict = keyval_parse("a.0=null,a.1=1", NULL, NULL, &error_abort);
     v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
     qobject_unref(qdict);
     visit_start_struct(v, NULL, NULL, 0, &error_abort);
diff --git a/util/keyval.c b/util/keyval.c
index 13def4af54..5929c9dba2 100644
--- a/util/keyval.c
+++ b/util/keyval.c
@@ -14,7 +14,7 @@ 
  * KEY=VALUE,... syntax:
  *
  *   key-vals     = [ key-val { ',' key-val } [ ',' ] ]
- *   key-val      = key '=' val
+ *   key-val      = 'help' | key '=' val
  *   key          = key-fragment { '.' key-fragment }
  *   key-fragment = / [^=,.]* /
  *   val          = { / [^,]* / | ',,' }
@@ -73,10 +73,14 @@ 
  *
  * Additional syntax for use with an implied key:
  *
- *   key-vals-ik  = val-no-key [ ',' key-vals ]
+ *   key-vals-ik  = 'help' | val-no-key [ ',' key-vals ]
  *   val-no-key   = / [^=,]* /
  *
  * where no-key is syntactic sugar for implied-key=val-no-key.
+ *
+ * 'help' for key-val or key-vals-ik is not interpreted as part of the
+ * JSON object, but instead requests help, which is a separate boolean
+ * output of the parser.
  */
 
 #include "qemu/osdep.h"
@@ -85,6 +89,7 @@ 
 #include "qapi/qmp/qlist.h"
 #include "qapi/qmp/qstring.h"
 #include "qemu/cutils.h"
+#include "qemu/help_option.h"
 #include "qemu/option.h"
 
 /*
@@ -166,7 +171,7 @@  static QObject *keyval_parse_put(QDict *cur,
  * On failure, return NULL.
  */
 static const char *keyval_parse_one(QDict *qdict, const char *params,
-                                    const char *implied_key,
+                                    const char *implied_key, bool *help,
                                     Error **errp)
 {
     const char *key, *key_end, *s, *end;
@@ -238,13 +243,20 @@  static const char *keyval_parse_one(QDict *qdict, const char *params,
     if (key == implied_key) {
         assert(!*s);
         s = params;
+    } else if (*s == '=') {
+        s++;
     } else {
-        if (*s != '=') {
+        if (is_help_option_n(key, s - key)) {
+            *help = true;
+            if (*s) {
+                s++;
+            }
+            return s;
+        } else {
             error_setg(errp, "Expected '=' after parameter '%.*s'",
                        (int)(s - key), key);
             return NULL;
         }
-        s++;
     }
 
     val = qstring_new();
@@ -260,6 +272,15 @@  static const char *keyval_parse_one(QDict *qdict, const char *params,
         qstring_append_chr(val, *s++);
     }
 
+    if (key == implied_key) {
+        const char *str_val = qstring_get_str(val);
+        if (is_help_option(str_val)) {
+            *help = true;
+            qobject_unref(val);
+            return s;
+        }
+    }
+
     if (!keyval_parse_put(cur, key_in_cur, val, key, key_end, errp)) {
         return NULL;
     }
@@ -388,21 +409,32 @@  static QObject *keyval_listify(QDict *cur, GSList *key_of_cur, Error **errp)
 
 /*
  * Parse @params in QEMU's traditional KEY=VALUE,... syntax.
+ *
  * If @implied_key, the first KEY= can be omitted.  @implied_key is
  * implied then, and VALUE can't be empty or contain ',' or '='.
+ *
+ * An option "help" or "?" without a value isn't added to the
+ * resulting dictionary, but instead is interpreted as requesting
+ * help. If @p_help is non-NULL, it is true on return if help was
+ * requested and false otherwise. All other options are parsed and
+ * returned normally so that context specific help can be printed.
+ *
+ * If @p_help is NULL and help is requested, an error is returned.
+ *
  * On success, return a dictionary of the parsed keys and values.
  * On failure, store an error through @errp and return NULL.
  */
 QDict *keyval_parse(const char *params, const char *implied_key,
-                    Error **errp)
+                    bool *p_help, Error **errp)
 {
     QDict *qdict = qdict_new();
     QObject *listified;
     const char *s;
+    bool help = false;
 
     s = params;
     while (*s) {
-        s = keyval_parse_one(qdict, s, implied_key, errp);
+        s = keyval_parse_one(qdict, s, implied_key, &help, errp);
         if (!s) {
             qobject_unref(qdict);
             return NULL;
@@ -410,6 +442,14 @@  QDict *keyval_parse(const char *params, const char *implied_key,
         implied_key = NULL;
     }
 
+    if (p_help) {
+        *p_help = help;
+    } else if (help) {
+        error_setg(errp, "Help is not available for this option");
+        qobject_unref(qdict);
+        return NULL;
+    }
+
     listified = keyval_listify(qdict, NULL, errp);
     if (!listified) {
         qobject_unref(qdict);