Re: [PATCH v3 1/4] qapi: Add a 'coroutine' flag for commands

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> This patch adds a new 'coroutine' flag to QMP command definitions that
> tells the QMP dispatcher that the command handler is safe to be run in a
> coroutine.
>
> Signed-off-by: Kevin Wolf <address@hidden>
> Reviewed-by: Marc-André Lureau <address@hidden>
> ---
>  tests/qapi-schema/qapi-schema-test.json |  1 +
>  docs/devel/qapi-code-gen.txt            |  4 ++++
>  include/qapi/qmp/dispatch.h             |  1 +
>  tests/test-qmp-cmds.c                   |  4 ++++
>  scripts/qapi/commands.py                | 17 +++++++++++------
>  scripts/qapi/doc.py                     |  2 +-
>  scripts/qapi/expr.py                    |  4 ++--
>  scripts/qapi/introspect.py              |  2 +-
>  scripts/qapi/schema.py                  |  9 ++++++---
>  tests/qapi-schema/qapi-schema-test.out  |  2 ++
>  tests/qapi-schema/test-qapi.py          |  7 ++++---
>  11 files changed, 37 insertions(+), 16 deletions(-)
>
> diff --git a/tests/qapi-schema/qapi-schema-test.json 
> b/tests/qapi-schema/qapi-schema-test.json
> index 9abf175fe0..1a850fe171 100644
> --- a/tests/qapi-schema/qapi-schema-test.json
> +++ b/tests/qapi-schema/qapi-schema-test.json
> @@ -147,6 +147,7 @@
>    'returns': 'UserDefTwo' }
>  
>  { 'command': 'cmd-success-response', 'data': {}, 'success-response': false }
> +{ 'command': 'coroutine-cmd', 'data': {}, 'coroutine': true }
>  
>  # Returning a non-dictionary requires a name from the whitelist
>  { 'command': 'guest-get-time', 'data': {'a': 'int', '*b': 'int' },
> diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
> index 45c93a43cc..753f6711d3 100644
> --- a/docs/devel/qapi-code-gen.txt
> +++ b/docs/devel/qapi-code-gen.txt
> @@ -457,6 +457,7 @@ Syntax:
>                  '*gen': false,
>                  '*allow-oob': true,
>                  '*allow-preconfig': true,
> +                '*coroutine': true,
>                  '*if': COND,
>                  '*features': FEATURES }
>  
> @@ -581,6 +582,9 @@ before the machine is built.  It defaults to false.  For 
> example:
>  QMP is available before the machine is built only when QEMU was
>  started with --preconfig.
>  
> +Member 'coroutine' tells the QMP dispatcher whether the command handler
> +is safe to be run in a coroutine. It defaults to false.

Two spaces after sentence-ending period, for consistency with the rest
of the file.

As discussed in review of prior versions, coroutine-safety is an
implementation detail that should not be exposed to management
applications.  Therefore, we want a flag, not a feature.

As far as I can tell, the new flag has no effect until PATCH 3 puts it
to use.  That's okay.

The doc update tells us when we may say 'coroutine': true, namely when
the handler function is coroutine-safe.  It doesn't quite tell us what
difference it makes, or rather will make after PATCH 3.  I think it
should.

In review of a prior version, Marc-André wondered whether keeping
allow-oob and coroutine separate makes sense.  Recall qapi-code-gen.txt:

    An OOB-capable command handler must satisfy the following conditions:

    - It terminates quickly.
    - It does not invoke system calls that may block.
    - It does not access guest RAM that may block when userfaultfd is
      enabled for postcopy live migration.
    - It takes only "fast" locks, i.e. all critical sections protected by
      any lock it takes also satisfy the conditions for OOB command
      handler code.

    The restrictions on locking limit access to shared state.  Such access
    requires synchronization, but OOB commands can't take the BQL or any
    other "slow" lock.

Kevin, does this rule out coroutine use?

> +
>  The optional 'if' member specifies a conditional.  See "Configuring
>  the schema" below for more on this.
>  
> diff --git a/include/qapi/qmp/dispatch.h b/include/qapi/qmp/dispatch.h
> index 9aa426a398..d6ce9efc8e 100644
> --- a/include/qapi/qmp/dispatch.h
> +++ b/include/qapi/qmp/dispatch.h
> @@ -24,6 +24,7 @@ typedef enum QmpCommandOptions
>      QCO_NO_SUCCESS_RESP       =  (1U << 0),
>      QCO_ALLOW_OOB             =  (1U << 1),
>      QCO_ALLOW_PRECONFIG       =  (1U << 2),
> +    QCO_COROUTINE             =  (1U << 3),
>  } QmpCommandOptions;
>  
>  typedef struct QmpCommand
> diff --git a/tests/test-qmp-cmds.c b/tests/test-qmp-cmds.c
> index 27b0afe55a..e2f71e42a1 100644
> --- a/tests/test-qmp-cmds.c
> +++ b/tests/test-qmp-cmds.c
> @@ -34,6 +34,10 @@ void qmp_cmd_success_response(Error **errp)
>  {
>  }
>  
> +void qmp_coroutine_cmd(Error **errp)
> +{
> +}
> +
>  Empty2 *qmp_user_def_cmd0(Error **errp)
>  {
>      return g_new0(Empty2, 1);
> diff --git a/scripts/qapi/commands.py b/scripts/qapi/commands.py
> index ab98e504f3..97e51401f1 100644
> --- a/scripts/qapi/commands.py
> +++ b/scripts/qapi/commands.py
> @@ -15,6 +15,7 @@ See the COPYING file in the top-level directory.
>  
>  from qapi.common import *
>  from qapi.gen import QAPIGenCCode, QAPISchemaModularCVisitor, ifcontext
> +from typing import List
>  
>  
>  def gen_command_decl(name, arg_type, boxed, ret_type):
> @@ -194,8 +195,9 @@ out:
>      return ret
>  
>  
> -def gen_register_command(name, success_response, allow_oob, allow_preconfig):
> -    options = []
> +def gen_register_command(name: str, success_response: bool, allow_oob: bool,
> +                         allow_preconfig: bool, coroutine: bool) -> str:
> +    options = [] # type: List[str]

Not sure such isolated type hints make sense.  I'd welcome patches to
explore systematic use of type hints.  Suggest to start with just one
module, so we can gauge effort and benefit before we jump in whole hog.

>  
>      if not success_response:
>          options += ['QCO_NO_SUCCESS_RESP']
> @@ -203,18 +205,20 @@ def gen_register_command(name, success_response, 
> allow_oob, allow_preconfig):
>          options += ['QCO_ALLOW_OOB']
>      if allow_preconfig:
>          options += ['QCO_ALLOW_PRECONFIG']
> +    if coroutine:
> +        options += ['QCO_COROUTINE']
>  
>      if not options:
>          options = ['QCO_NO_OPTIONS']
>  
> -    options = " | ".join(options)
> +    options_str = " | ".join(options)
>  
>      ret = mcgen('''
>      qmp_register_command(cmds, "%(name)s",
>                           qmp_marshal_%(c_name)s, %(opts)s);
>  ''',
>                  name=name, c_name=c_name(name),
> -                opts=options)
> +                opts=options_str)
>      return ret
>  
>  

Some extra churn due to type hints here.  Distracting.  Suggest not to
mix adding type hints to existing code with feature work.

> @@ -278,7 +282,7 @@ void %(c_prefix)sqmp_init_marshal(QmpCommandList *cmds);
>  
>      def visit_command(self, name, info, ifcond, arg_type, ret_type, gen,
>                        success_response, boxed, allow_oob, allow_preconfig,
> -                      features):
> +                      coroutine, features):
>          if not gen:
>              return
>          # FIXME: If T is a user-defined type, the user is responsible
> @@ -296,7 +300,8 @@ void %(c_prefix)sqmp_init_marshal(QmpCommandList *cmds);
>              self._genh.add(gen_marshal_decl(name))
>              self._genc.add(gen_marshal(name, arg_type, boxed, ret_type))
>              self._regy.add(gen_register_command(name, success_response,
> -                                                allow_oob, allow_preconfig))
> +                                                allow_oob, allow_preconfig,
> +                                                coroutine))
>  
>  
>  def gen_commands(schema, output_dir, prefix):
> diff --git a/scripts/qapi/doc.py b/scripts/qapi/doc.py
> index 6f1c17f71f..8b6978c81e 100644
> --- a/scripts/qapi/doc.py
> +++ b/scripts/qapi/doc.py
> @@ -265,7 +265,7 @@ class QAPISchemaGenDocVisitor(QAPISchemaVisitor):
>  
>      def visit_command(self, name, info, ifcond, arg_type, ret_type, gen,
>                        success_response, boxed, allow_oob, allow_preconfig,
> -                      features):
> +                      coroutine, features):
>          doc = self.cur_doc
>          self._gen.add(texi_msg('Command', doc, ifcond,
>                                 texi_arguments(doc,
> diff --git a/scripts/qapi/expr.py b/scripts/qapi/expr.py
> index d7a289eded..9dbfa3edf0 100644
> --- a/scripts/qapi/expr.py
> +++ b/scripts/qapi/expr.py
> @@ -89,7 +89,7 @@ def check_flags(expr, info):
>          if key in expr and expr[key] is not False:
>              raise QAPISemError(
>                  info, "flag '%s' may only use false value" % key)
> -    for key in ['boxed', 'allow-oob', 'allow-preconfig']:
> +    for key in ['boxed', 'allow-oob', 'allow-preconfig', 'coroutine']:
>          if key in expr and expr[key] is not True:
>              raise QAPISemError(
>                  info, "flag '%s' may only use true value" % key)
> @@ -344,7 +344,7 @@ def check_exprs(exprs):
>                         ['command'],
>                         ['data', 'returns', 'boxed', 'if', 'features',
>                          'gen', 'success-response', 'allow-oob',
> -                        'allow-preconfig'])
> +                        'allow-preconfig', 'coroutine'])
>              normalize_members(expr.get('data'))
>              check_command(expr, info)
>          elif meta == 'event':
> diff --git a/scripts/qapi/introspect.py b/scripts/qapi/introspect.py
> index b3a463dd8b..8a296a69d6 100644
> --- a/scripts/qapi/introspect.py
> +++ b/scripts/qapi/introspect.py
> @@ -212,7 +212,7 @@ const QLitObject %(c_name)s = %(c_string)s;
>  
>      def visit_command(self, name, info, ifcond, arg_type, ret_type, gen,
>                        success_response, boxed, allow_oob, allow_preconfig,
> -                      features):
> +                      coroutine, features):
>          arg_type = arg_type or self._schema.the_empty_object_type
>          ret_type = ret_type or self._schema.the_empty_object_type
>          obj = {'arg-type': self._use_type(arg_type),
> diff --git a/scripts/qapi/schema.py b/scripts/qapi/schema.py
> index cf0045f34e..753bf233a6 100644
> --- a/scripts/qapi/schema.py
> +++ b/scripts/qapi/schema.py
> @@ -128,7 +128,7 @@ class QAPISchemaVisitor(object):
>  
>      def visit_command(self, name, info, ifcond, arg_type, ret_type, gen,
>                        success_response, boxed, allow_oob, allow_preconfig,
> -                      features):
> +                      coroutine, features):
>          pass
>  
>      def visit_event(self, name, info, ifcond, arg_type, boxed):
> @@ -678,7 +678,7 @@ class QAPISchemaCommand(QAPISchemaEntity):
>  
>      def __init__(self, name, info, doc, ifcond, arg_type, ret_type,
>                   gen, success_response, boxed, allow_oob, allow_preconfig,
> -                 features):
> +                 coroutine, features):
>          QAPISchemaEntity.__init__(self, name, info, doc, ifcond, features)
>          assert not arg_type or isinstance(arg_type, str)
>          assert not ret_type or isinstance(ret_type, str)
> @@ -691,6 +691,7 @@ class QAPISchemaCommand(QAPISchemaEntity):
>          self.boxed = boxed
>          self.allow_oob = allow_oob
>          self.allow_preconfig = allow_preconfig
> +        self.coroutine = coroutine
>  
>      def check(self, schema):
>          QAPISchemaEntity.check(self, schema)
> @@ -732,7 +733,7 @@ class QAPISchemaCommand(QAPISchemaEntity):
>                                self.arg_type, self.ret_type,
>                                self.gen, self.success_response,
>                                self.boxed, self.allow_oob,
> -                              self.allow_preconfig,
> +                              self.allow_preconfig, self.coroutine,
>                                self.features)
>  
>  
> @@ -1014,6 +1015,7 @@ class QAPISchema(object):
>          boxed = expr.get('boxed', False)
>          allow_oob = expr.get('allow-oob', False)
>          allow_preconfig = expr.get('allow-preconfig', False)
> +        coroutine = expr.get('coroutine', False)
>          ifcond = expr.get('if')
>          features = expr.get('features', [])
>          if isinstance(data, OrderedDict):
> @@ -1025,6 +1027,7 @@ class QAPISchema(object):
>          self._def_entity(QAPISchemaCommand(name, info, doc, ifcond, data, 
> rets,
>                                             gen, success_response,
>                                             boxed, allow_oob, allow_preconfig,
> +                                           coroutine,
>                                             self._make_features(features, 
> info)))
>  
>      def _def_event(self, expr, info, doc):
> diff --git a/tests/qapi-schema/qapi-schema-test.out 
> b/tests/qapi-schema/qapi-schema-test.out
> index 3660e75a48..51bfe8bfc7 100644
> --- a/tests/qapi-schema/qapi-schema-test.out
> +++ b/tests/qapi-schema/qapi-schema-test.out
> @@ -217,6 +217,8 @@ command user_def_cmd2 q_obj_user_def_cmd2-arg -> 
> UserDefTwo
>      gen=True success_response=True boxed=False oob=False preconfig=False
>  command cmd-success-response None -> None
>      gen=True success_response=False boxed=False oob=False preconfig=False
> +command coroutine-cmd None -> None
> +    gen=True success_response=True boxed=False oob=False preconfig=False 
> coroutine=True
>  object q_obj_guest-get-time-arg
>      member a: int optional=False
>      member b: int optional=True
> diff --git a/tests/qapi-schema/test-qapi.py b/tests/qapi-schema/test-qapi.py
> index bad14edb47..7a8e65188d 100755
> --- a/tests/qapi-schema/test-qapi.py
> +++ b/tests/qapi-schema/test-qapi.py
> @@ -70,12 +70,13 @@ class QAPISchemaTestVisitor(QAPISchemaVisitor):
>  
>      def visit_command(self, name, info, ifcond, arg_type, ret_type, gen,
>                        success_response, boxed, allow_oob, allow_preconfig,
> -                      features):
> +                      coroutine, features):
>          print('command %s %s -> %s'
>                % (name, arg_type and arg_type.name,
>                   ret_type and ret_type.name))
> -        print('    gen=%s success_response=%s boxed=%s oob=%s preconfig=%s'
> -              % (gen, success_response, boxed, allow_oob, allow_preconfig))
> +        print('    gen=%s success_response=%s boxed=%s oob=%s preconfig=%s%s'
> +              % (gen, success_response, boxed, allow_oob, allow_preconfig,
> +                 " coroutine=True" if coroutine else ""))
>          self._print_if(ifcond)
>          self._print_features(features)

Looks sane to me.