From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 558E2D0C61F for ; Fri, 25 Oct 2024 15:18:03 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1t4M3y-0002Hu-Uw; Fri, 25 Oct 2024 11:17:18 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1t4M3x-0002HW-Em for qemu-devel@nongnu.org; Fri, 25 Oct 2024 11:17:17 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1t4M3v-0007IA-Mi for qemu-devel@nongnu.org; Fri, 25 Oct 2024 11:17:17 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1729869434; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ogf30TefIp5A00FtClSfbDYW5v471Ll2hDynpcL1WSU=; b=bCpDFDOUofcTrjyFT881RYyHUKJ00QNxnmDCjPuu3kyRf7f4WTOsXDWWDpLkugWO575seR y3zyXCSK99BMdF7AngCv573kIi+wyhMF6r6gWP9fIAdNBXx0sZbBMT7nn5o/txeWSmbolL qTOrVSVV6x8I3yUxevbMotsyCfliB8I= Received: from mail-qv1-f69.google.com (mail-qv1-f69.google.com [209.85.219.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-668-PUHrUEInMyWNt9PxpJgUpg-1; Fri, 25 Oct 2024 11:17:11 -0400 X-MC-Unique: PUHrUEInMyWNt9PxpJgUpg-1 Received: by mail-qv1-f69.google.com with SMTP id 6a1803df08f44-6cbf039dccfso28362426d6.0 for ; Fri, 25 Oct 2024 08:17:11 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1729869430; x=1730474230; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=ogf30TefIp5A00FtClSfbDYW5v471Ll2hDynpcL1WSU=; b=vzV5ZjnNrE6oXNiPAsjPY2j+QBGpId3k52duQj8w/rEYNkDHhrWMpPARgaQ+pVqDzr idxVBojfUYuOUriAv5ksHDozp2+Lct8xDSnDKqBX7HtmyUOnsVCh7TqJsh8GEJIjYtpV 6xvArGaEoz7/944o+SiMnBowpSNHxgvhFnqlharEEtx8vuWBlFAERwn01tefFtkFYIwp XLEkwViOYQUvmd6RiqVXYF7cN4xofhuXLd4KFOGDIW6G2kI9yYJwsbmpm1bSR/4vdk5W 73f/Cj6PSEOvy34QvPUXw9p/hV672U+Dk4DWIgub/iX+YUJTrYN8uusUB+KCNPsicnRH Iwrg== X-Gm-Message-State: AOJu0YwD/HmmVdGipLZEmioICEJ+s+kPGAUHkIy0td1IfpQdeh2FMcGX SNbgcSjxjkgngwaVjDaCmd+39mS8q9iYLVUv6kMCj865sUMPB96b1vSKPqCHeCH2riiUrHTPADa 6lplF2QGtSshh+iRNgoLbMhPh3FGrunF3a8WiR3TX9RsM1AKrf5yu X-Received: by 2002:a05:6214:5a03:b0:6cb:ef96:c79e with SMTP id 6a1803df08f44-6d09268cbecmr67993226d6.34.1729869430449; Fri, 25 Oct 2024 08:17:10 -0700 (PDT) X-Google-Smtp-Source: AGHT+IG8BYvDxtGkLfCytcNcNsS7QCvaxRARwYpdX8lt6P0YzQDQUFRcKK0QFYtLsdLIwrpJzCFIEA== X-Received: by 2002:a05:6214:5a03:b0:6cb:ef96:c79e with SMTP id 6a1803df08f44-6d09268cbecmr67992766d6.34.1729869430018; Fri, 25 Oct 2024 08:17:10 -0700 (PDT) Received: from x1n (pool-99-254-114-190.cpe.net.cable.rogers.com. [99.254.114.190]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6d1799fd518sm6449486d6.102.2024.10.25.08.17.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 25 Oct 2024 08:17:09 -0700 (PDT) Date: Fri, 25 Oct 2024 11:17:04 -0400 From: Peter Xu To: Markus Armbruster Cc: qemu-devel@nongnu.org, Fabiano Rosas , Igor Mammedov , Juraj Marcin , "Michael S . Tsirkin" , "Dr . David Alan Gilbert" , =?utf-8?Q?C=C3=A9dric?= Le Goater , Eduardo Habkost , Daniel P =?utf-8?B?LiBCZXJyYW5nw6k=?= , Alex Williamson , Paolo Bonzini , Peter Maydell Subject: Re: [PATCH 1/4] qom: TYPE_SINGLETON interface Message-ID: References: <20241024165627.1372621-1-peterx@redhat.com> <20241024165627.1372621-2-peterx@redhat.com> <87plnoli5c.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87plnoli5c.fsf@pond.sub.org> Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.454, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Fri, Oct 25, 2024 at 10:07:59AM +0200, Markus Armbruster wrote: > Peter Xu writes: > > > Signed-off-by: Peter Xu > > --- > > include/qom/object_interfaces.h | 47 +++++++++++++++++++++++++++++++++ > > qom/object.c | 3 +++ > > qom/object_interfaces.c | 24 +++++++++++++++++ > > qom/qom-qmp-cmds.c | 22 ++++++++++++--- > > system/qdev-monitor.c | 7 +++++ > > 5 files changed, 100 insertions(+), 3 deletions(-) > > > > diff --git a/include/qom/object_interfaces.h b/include/qom/object_interfaces.h > > index 02b11a7ef0..9b2cc0e554 100644 > > --- a/include/qom/object_interfaces.h > > +++ b/include/qom/object_interfaces.h > > @@ -177,4 +177,51 @@ bool user_creatable_del(const char *id, Error **errp); > > */ > > void user_creatable_cleanup(void); > > > > +#define TYPE_SINGLETON "singleton" > > + > > +typedef struct SingletonClass SingletonClass; > > +DECLARE_CLASS_CHECKERS(SingletonClass, SINGLETON, TYPE_SINGLETON) > > + > > +/** > > + * SingletonClass: > > + * > > + * @parent_class: the base class > > + * @get_instance: fetch the singleton instance if it is created, > > + * NULL otherwise. > > + * > > + * Singleton class describes the type of object classes that can only > > + * provide one instance for the whole lifecycle of QEMU. It will fail the > > + * operation if one attemps to create more than one instance. > > + * > > + * One can fetch the single object using class's get_instance() callback if > > + * it was created before. This can be useful for operations like QMP > > + * qom-list-properties, where dynamically creating an object might not be > > + * feasible. > > + */ > > +struct SingletonClass { > > + /* */ > > + InterfaceClass parent_class; > > + /* */ > > + Object *(*get_instance)(Error **errp); > > +}; > > + > > +/** > > + * object_class_is_singleton: > > + * > > + * @class: the class to detect singleton > > + * > > + * Returns: true if it's a singleton class, false otherwise. > > + */ > > +bool object_class_is_singleton(ObjectClass *class); > > + > > +/** > > + * singleton_get_instance: > > + * > > + * @class: the class to fetch singleton instance > > + * > > + * Returns: the object* if the class is a singleton class and the singleton > > + * object is created, NULL otherwise. > > + */ > > +Object *singleton_get_instance(ObjectClass *class); > > A non-null return value can become dangling when the object gets > destroyed. This could conceivably happen in another thread. > > The obviously safe interface would take a reference the caller must > unref when done. Ouch, thanks for spotting this! Yes we definitely need a refcount at least.. > > > + > > #endif > > diff --git a/qom/object.c b/qom/object.c > > index 11424cf471..ded299ae1a 100644 > > --- a/qom/object.c > > +++ b/qom/object.c > > @@ -553,6 +553,9 @@ static void object_initialize_with_type(Object *obj, size_t size, TypeImpl *type > > g_assert(type->abstract == false); > > g_assert(size >= type->instance_size); > > > > + /* Singleton class can only create one object */ > > + g_assert(!singleton_get_instance(type->class)); > > + > > memset(obj, 0, type->instance_size); > > obj->class = type->class; > > object_ref(obj); > > diff --git a/qom/object_interfaces.c b/qom/object_interfaces.c > > index e0833c8bfe..6766060d0a 100644 > > --- a/qom/object_interfaces.c > > +++ b/qom/object_interfaces.c > > @@ -354,6 +354,23 @@ void user_creatable_cleanup(void) > > object_unparent(object_get_objects_root()); > > } > > > > +bool object_class_is_singleton(ObjectClass *class) > > +{ > > + return !!object_class_dynamic_cast(class, TYPE_SINGLETON); > > +} > > + > > +Object *singleton_get_instance(ObjectClass *class) > > +{ > > + SingletonClass *singleton = > > + (SingletonClass *)object_class_dynamic_cast(class, TYPE_SINGLETON); > > + > > + if (!singleton) { > > + return NULL; > > + } > > + > > + return singleton->get_instance(&error_abort); > > +} > > + > > static void register_types(void) > > { > > static const TypeInfo uc_interface_info = { > > @@ -362,7 +379,14 @@ static void register_types(void) > > .class_size = sizeof(UserCreatableClass), > > }; > > > > + static const TypeInfo singleton_interface_info = { > > + .name = TYPE_SINGLETON, > > + .parent = TYPE_INTERFACE, > > + .class_size = sizeof(SingletonClass), > > + }; > > + > > type_register_static(&uc_interface_info); > > + type_register_static(&singleton_interface_info); > > } > > > > type_init(register_types) > > diff --git a/qom/qom-qmp-cmds.c b/qom/qom-qmp-cmds.c > > index e91a235347..ecc1cf781c 100644 > > --- a/qom/qom-qmp-cmds.c > > +++ b/qom/qom-qmp-cmds.c > > @@ -126,6 +126,7 @@ ObjectPropertyInfoList *qmp_device_list_properties(const char *typename, > > ObjectProperty *prop; > > ObjectPropertyIterator iter; > > ObjectPropertyInfoList *prop_list = NULL; > > + bool create; > > > > klass = module_object_class_by_name(typename); > > if (klass == NULL) { > > @@ -141,7 +142,15 @@ ObjectPropertyInfoList *qmp_device_list_properties(const char *typename, > > return NULL; > > } > > > > - obj = object_new(typename); > > + /* Avoid creating multiple instances if the class is a singleton */ > > + create = !object_class_is_singleton(klass) || > > + !singleton_get_instance(klass); > > + > > + if (create) { > > + obj = object_new(typename); > > If the class is not a singleton or else if no instance exists, we create > a temporary instance. > > > + } else { > > + obj = singleton_get_instance(klass); > > If the class is a singleton and the instance exists, we use that > instead. > > Any properties the instance has created dynamically after object_new() > are visible to introspection. This is not the case when we create a > temporary instance. Such subtle differences are problematic. If we > decide we're okay with this one, we need to document it. I am thinking what's the major use case for device-list-properties. If it's for mgmt to query a device property list for set/get before operating on a real instance (which represents already somewhere in the device tree), I hope it's fine. Basically, it means whatever queried here can always be used with qom-get/qom-set for this singleton as long as it's already created and present. I hope Libvirt cannot cache this results anyway, because we already have: ## # @device-list-properties: # ... # .. note:: Objects can create properties at runtime, for example to # describe links between different devices and/or objects. These # properties are not included in the output of this command. So I think it means mgmt cannot cache these results, but only query before qom-set/qom-get to make sure it's valid. In that case, maybe we can change that to s/are not included/may not be included/? > > > + } > > > > object_property_iter_init(&iter, obj); > > while ((prop = object_property_iter_next(&iter))) { > > @@ -172,7 +181,9 @@ ObjectPropertyInfoList *qmp_device_list_properties(const char *typename, > > QAPI_LIST_PREPEND(prop_list, info); > > } > > > > - object_unref(obj); > > + if (create) { > > + object_unref(obj); > > + } > > > > return prop_list; > > } > > @@ -199,7 +210,12 @@ ObjectPropertyInfoList *qmp_qom_list_properties(const char *typename, > > return NULL; > > } > > > > - if (object_class_is_abstract(klass)) { > > + /* > > + * Abstract classes are not for instantiations, meanwhile avoid > > + * creating temporary singleton objects because it can cause conflicts > > + * if there's already one created. > > + */ > > + if (object_class_is_abstract(klass) || object_class_is_singleton(klass)) { > > object_class_property_iter_init(&iter, klass); > > } else { > > obj = object_new(typename); > > If the class is a singleton, we treat it as if it was abstract. Its > instance properties, if any, are not visible in qom-list-properties. > This is a defect. If you make an existing class with instance > properties a singleton, the defect is a regression. I can switch to get_instance() for singleton when it's available. Would that work? Thanks! > > > diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c > > index 44994ea0e1..1310f35c9f 100644 > > --- a/system/qdev-monitor.c > > +++ b/system/qdev-monitor.c > > @@ -36,6 +36,7 @@ > > #include "qemu/option.h" > > #include "qemu/qemu-print.h" > > #include "qemu/option_int.h" > > +#include "qom/object_interfaces.h" > > #include "sysemu/block-backend.h" > > #include "migration/misc.h" > > #include "qemu/cutils.h" > > @@ -643,6 +644,12 @@ DeviceState *qdev_device_add_from_qdict(const QDict *opts, > > return NULL; > > } > > > > + if (singleton_get_instance(OBJECT_CLASS(dc))) { > > + error_setg(errp, "Class '%s' only supports one instance", > > + driver); > > + return NULL; > > + } > > + > > /* find bus */ > > path = qdict_get_try_str(opts, "bus"); > > if (path != NULL) { > -- Peter Xu