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spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:38692 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lV5gE-0003tq-IU for qemu-devel@archiver.kernel.org; Sat, 10 Apr 2021 00:57:10 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60006) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lV5fM-0003Ol-5L for qemu-devel@nongnu.org; Sat, 10 Apr 2021 00:56:16 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:38883) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lV5fK-0006BL-6V for qemu-devel@nongnu.org; Sat, 10 Apr 2021 00:56:15 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1618030573; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=HehfS8YfIcLflDZM9bjP+jUrvDfmQWop4v9u4wYK/dA=; b=PpOWa5mbDkM64jetorwvJfz6m9rvrfjSbQRez6Dey9MeVW/yo63Kzl8TccSP7EXt1p8vIt PKnprq7jSNTOHF3xuOyrAVjayCenhyoerGlRfmj+YJHgPvFAgMJZ4pp7Wo1ZokwupW014z bYhVnGK2/kRvHatgDwlFyeq4J9l+JG0= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-589-tgBah-IFNSyWHY4gFS0bBw-1; Sat, 10 Apr 2021 00:56:10 -0400 X-MC-Unique: tgBah-IFNSyWHY4gFS0bBw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 19E8C1883520; Sat, 10 Apr 2021 04:56:09 +0000 (UTC) Received: from thuth.remote.csb (ovpn-112-20.ams2.redhat.com [10.36.112.20]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3AC675D768; Sat, 10 Apr 2021 04:56:06 +0000 (UTC) Subject: Re: [PATCH 1/2] qdev: Separate implementations of qdev_get_machine() for user and system To: Greg Kurz , qemu-devel@nongnu.org References: <20210409160339.500167-1-groug@kaod.org> <20210409160339.500167-2-groug@kaod.org> From: Thomas Huth Message-ID: Date: Sat, 10 Apr 2021 06:56:06 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <20210409160339.500167-2-groug@kaod.org> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=thuth@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paolo Bonzini , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , Eduardo Habkost , Markus Armbruster Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 09/04/2021 18.03, Greg Kurz wrote: > Despite its simple name and common usage of "getting a pointer to > the machine" in system-mode emulation, qdev_get_machine() has some > subtilities. > > First, it can be called when running user-mode emulation : this is > because user-mode partly relies on qdev to instantiate its CPU > model. > > Second, but not least, it has a side-effect : if it cannot find an > object at "/machine" in the QOM tree, it creates a dummy "container" > object and put it there. A simple check on the type returned by > qdev_get_machine() allows user-mode to run the common qdev code, > skipping the parts that only make sense for system-mode. > > This side-effect turns out to complicate the use of qdev_get_machine() > for the system-mode case though. Most notably, qdev_get_machine() must > not be called before the machine object is added to the QOM tree by > qemu_create_machine(), otherwise the existing dummy "container" object > would cause qemu_create_machine() to fail with something like : > > Unexpected error in object_property_try_add() at ../../qom/object.c:1223: > qemu-system-ppc64: attempt to add duplicate property 'machine' to > object (type 'container') > Aborted (core dumped) > > This situation doesn't exist in the current code base, mostly because > of preventive fixing of some "latent bugs" in QEMU 4.0 (see 1a3ec8c1564 > and e2fb3fbbf9c for details). > > A new kind of breakage was spotted very recently though : > > $ ./qemu-system-ppc64 -device power8_v2.0-spapr-cpu-core,help > /home/thuth/devel/qemu/include/hw/boards.h:24: > MACHINE: Object 0x5635bd53af10 is not an instance of type machine > Aborted (core dumped) > > This comes from the change 3df261b6676b in QEMU 5.0. It unwillingly > added a new condition for qdev_get_machine() to be called too early, > breaking MACHINE(qdev_get_machine()) in generic cpu-core code this > time. > > In order to avoid further subtle breakages like this, change the > implentation of qdev_get_machine() to: > - keep the existing behaviour of creating the dummy "container" > object for the user-mode case only ; > - abort() if the machine doesn't exist yet in the QOM tree for > the system-mode case. This gives a precise hint to developpers > that calling qdev_get_machine() too early is a programming bug. > > This is achieved with a new do_qdev_get_machine() function called > from qdev_get_machine(), with different implementations for system > and user mode. > > $ ./qemu-system-ppc64 -device power8_v2.0-spapr-cpu-core,help > qemu-system-ppc64: ../../hw/core/machine.c:1290: > qdev_get_machine: Assertion `machine != NULL' failed. > Aborted (core dumped) > > Reported-by: Thomas Huth > Signed-off-by: Greg Kurz > --- > hw/core/machine.c | 14 ++++++++++++++ > hw/core/qdev.c | 2 +- > include/hw/qdev-core.h | 1 + > stubs/meson.build | 1 + > stubs/qdev-get-machine.c | 11 +++++++++++ > 5 files changed, 28 insertions(+), 1 deletion(-) > create mode 100644 stubs/qdev-get-machine.c > > diff --git a/hw/core/machine.c b/hw/core/machine.c > index 40def78183a7..fecca4023105 100644 > --- a/hw/core/machine.c > +++ b/hw/core/machine.c > @@ -1293,6 +1293,20 @@ void qdev_machine_creation_done(void) > register_global_state(); > } > > +Object *do_qdev_get_machine(void) > +{ > + Object *machine; > + > + machine = object_resolve_path_component(object_get_root(), "machine"); > + /* > + * qdev_get_machine() shouldn't be called before qemu_create_machine() > + * has created the "/machine" path. > + */ > + assert(machine != NULL); > + > + return machine; > +} > + > static const TypeInfo machine_info = { > .name = TYPE_MACHINE, > .parent = TYPE_OBJECT, > diff --git a/hw/core/qdev.c b/hw/core/qdev.c > index cefc5eaa0a92..1122721b2bf0 100644 > --- a/hw/core/qdev.c > +++ b/hw/core/qdev.c > @@ -1131,7 +1131,7 @@ Object *qdev_get_machine(void) > static Object *dev; > > if (dev == NULL) { > - dev = container_get(object_get_root(), "/machine"); > + dev = do_qdev_get_machine(); > } > > return dev; > diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h > index bafc311bfa1b..90e295e0bc1a 100644 > --- a/include/hw/qdev-core.h > +++ b/include/hw/qdev-core.h > @@ -782,6 +782,7 @@ const char *qdev_fw_name(DeviceState *dev); > > void qdev_assert_realized_properly(void); > Object *qdev_get_machine(void); > +Object *do_qdev_get_machine(void); > > /* FIXME: make this a link<> */ > bool qdev_set_parent_bus(DeviceState *dev, BusState *bus, Error **errp); > diff --git a/stubs/meson.build b/stubs/meson.build > index be6f6d609e58..b99ee2b33e94 100644 > --- a/stubs/meson.build > +++ b/stubs/meson.build > @@ -54,3 +54,4 @@ if have_system > else > stub_ss.add(files('qdev.c')) > endif > +stub_ss.add(files('qdev-get-machine.c')) > diff --git a/stubs/qdev-get-machine.c b/stubs/qdev-get-machine.c > new file mode 100644 > index 000000000000..ed4cdaa01900 > --- /dev/null > +++ b/stubs/qdev-get-machine.c > @@ -0,0 +1,11 @@ > +#include "qemu/osdep.h" > +#include "hw/qdev-core.h" > + > +Object *do_qdev_get_machine(void) > +{ > + /* > + * This will create a "container" and add it to the QOM tree, if there > + * isn't one already. > + */ > + return container_get(object_get_root(), "/machine"); > +} > Reviewed-by: Thomas Huth ... but I think I agree with Eduardo, we should likely add this only after 6.0 has been released.