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Sat, 17 Feb 2024 21:47:30 -0800 (PST) Received: from [157.82.207.250] ([157.82.207.250]) by smtp.gmail.com with ESMTPSA id l184-20020a6391c1000000b005dca5caed40sm2384968pge.81.2024.02.17.21.47.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 17 Feb 2024 21:47:29 -0800 (PST) Message-ID: Date: Sun, 18 Feb 2024 14:47:25 +0900 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] qdev: Report an error for machine without HotplugHandler To: Markus Armbruster Cc: Paolo Bonzini , =?UTF-8?Q?Daniel_P=2E_Berrang=C3=A9?= , Eduardo Habkost , qemu-devel@nongnu.org References: <20231210-bus-v2-1-34ebf5726fa0@daynix.com> <87h6kpgrl7.fsf@pond.sub.org> <8734vzsj6k.fsf@pond.sub.org> <87y1dpgvim.fsf@pond.sub.org> Content-Language: en-US From: Akihiko Odaki In-Reply-To: <87y1dpgvim.fsf@pond.sub.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: none client-ip=2001:4860:4864:20::36; envelope-from=akihiko.odaki@daynix.com; helo=mail-oa1-x36.google.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_NONE=0.001, T_SCC_BODY_TEXT_LINE=-0.01 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 2023/12/20 16:53, Markus Armbruster wrote: > Akihiko Odaki writes: > >> On 2023/12/18 23:02, Markus Armbruster wrote: >>> Akihiko Odaki writes: >>> >>>> On 2023/12/11 15:51, Markus Armbruster wrote: >>>>> Akihiko Odaki writes: >>>>> >>>>>> The HotplugHandler of the machine will be used when the parent bus does >>>>>> not exist, but the machine may not have one. Report an error in such a >>>>>> case instead of aborting. >>>>>> >>>>>> Fixes: 7716b8ca74 ("qdev: HotplugHandler: Add support for unplugging BUS-less devices") >>>>>> Signed-off-by: Akihiko Odaki >>>>> >>>>> Do you have a reproducer for the crash? >>>>> >>>>>> --- >>>>>> Changes in v2: >>>>>> - Fixed indention. >>>>>> - Link to v1: https://lore.kernel.org/r/20231202-bus-v1-1-f7540e3a8d62@daynix.com >>>>>> --- >>>>>> system/qdev-monitor.c | 13 ++++++++++--- >>>>>> 1 file changed, 10 insertions(+), 3 deletions(-) >>>>>> >>>>>> diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c >>>>>> index a13db763e5..5fe5d49c20 100644 >>>>>> --- a/system/qdev-monitor.c >>>>>> +++ b/system/qdev-monitor.c >>>>>> @@ -927,9 +927,16 @@ void qdev_unplug(DeviceState *dev, Error **errp) >>>>> void qdev_unplug(DeviceState *dev, Error **errp) >>>>> { >>>>> DeviceClass *dc = DEVICE_GET_CLASS(dev); >>>>> HotplugHandler *hotplug_ctrl; >>>>> HotplugHandlerClass *hdc; >>>>> Error *local_err = NULL; >>>>> if (qdev_unplug_blocked(dev, errp)) { >>>>> return; >>>>> } >>>>> if (dev->parent_bus && !qbus_is_hotpluggable(dev->parent_bus)) { >>>>> error_setg(errp, QERR_BUS_NO_HOTPLUG, dev->parent_bus->name); >>>>> return; >>>>> } >>>>> if (!dc->hotpluggable) { >>>>> error_setg(errp, QERR_DEVICE_NO_HOTPLUG, >>>>> object_get_typename(OBJECT(dev))); >>>>> return; >>>>> } >>>>> if (!migration_is_idle() && !dev->allow_unplug_during_migration) { >>>>> error_setg(errp, "device_del not allowed while migrating"); >>>>> return; >>>>> } >>>>> >>>>>> qdev_hot_removed = true; >>>>>> hotplug_ctrl = qdev_get_hotplug_handler(dev); >>>>>> - /* hotpluggable device MUST have HotplugHandler, if it doesn't >>>>>> - * then something is very wrong with it */ >>>>>> - g_assert(hotplug_ctrl); >>>>>> + if (!hotplug_ctrl) { >>>>>> + /* >>>>>> + * hotpluggable bus MUST have HotplugHandler, if it doesn't >>>>>> + * then something is very wrong with it >>>>>> + */ >>>>>> + assert(!dev->parent_bus); >>>>>> + >>>>>> + error_setg(errp, "The machine does not support hotplugging for a device without parent bus"); >>>>>> + return; >>>>>> + } >>>>> >>>>> Extended version of my question above: what are the devices where >>>>> qdev_get_hotplug_handler(dev) returns null here? >>>> >>>> Start a VM: qemu-system-aarch64 -M virt -nographic >>>> Run the following on its HMP: device_del /machine/unattached/device[0] >>>> >>>> It tries to unplug cortex-a15-arm-cpu and crashes. >>> >>> This device has no parent bus (dev->parent_bus is null), but is marked >>> hot-pluggable (dc->hotpluggable is true). Question for somebody >>> familiar with the hot-plug machinery: is this sane? >> >> Setting hotpluggable false for each device without bus_type gives the same effect, but is error-prone. > > Having hotpluggable = true when the device cannot be hot-plugged is > *wrong*. You might be able to paper over the wrongness so the code > works anyway, but nothing good can come out of lying to developers > trying to understand how the code works. Hi, I'm now revisiting this patch and now I think it is still semantically correct. This patch indeed prevents hotplugging a hotpluggable device and that may sound irrational. However, we should note that the entity that prevents hotplugging is not the device, but the machine that lacks a hotplug handler. So we can say the device itself is hotpluggable, but the machine is preventing hotplugging. We already do similar in a case that a device has a parent bus. qbus_is_hotpluggable() returns false if the parent bus lacks a hotplug handler and prevents from hotplugging a hotpluggable device. The device class must still have hotpluggable = true in such a case because another instance of device may be plugged into a bus that has a hotplug handler. I'll submit v3 soon so please check if this reasoning sounds valid for you and review it once I submit it. Regards, Akihiko Odaki