Hi Markus, On Wed, Dec 20, 2023 at 08:53:21AM +0100, Markus Armbruster wrote: > Date: Wed, 20 Dec 2023 08:53:21 +0100 > From: Markus Armbruster <arm...@redhat.com> > Subject: Re: [PATCH v2] qdev: Report an error for machine without > HotplugHandler > > Akihiko Odaki <akihiko.od...@daynix.com> writes: > > > On 2023/12/18 23:02, Markus Armbruster wrote: > >> Akihiko Odaki <akihiko.od...@daynix.com> writes: > >> > >>> On 2023/12/11 15:51, Markus Armbruster wrote: > >>>> Akihiko Odaki <akihiko.od...@daynix.com> 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 <akihiko.od...@daynix.com> > >>>> > >>>> 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-f7540e3a8...@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. > > Three ideas to avoid the lying: > > 1. default hotpluggable to bus_type != NULL. > > 2. assert(dc->bus_type || !dc->hotpluggable) in a suitable spot. > > 3. Change the meaning of hotpluggable, and rename it to reflect its new > meaning. Requires a careful reading of its uses. I wouldn't go there. >
What about 4 (or maybe 3.1) - droping this hotpluggable flag and just use a helper (like qbus) to check if device is hotpluggable? This removes the confusion of that flag and also reduces the number of configuration items for DeviceState that require developer attention. A simple helper is as follows: static inline bool qdev_is_hotpluggable(DeviceState *dev) { /* * Many Machines don't implement qdev_hotplug_allowed(). * * TODO: Once all of them complete missing qdev_hotplug_allowed(), * use qdev_hotplug_allowed() here. */ bool hotpluggable = !!qdev_get_machine_hotplug_handler(dev); if (!hotpluggable && dev->parent_bus) { hotpluggable = qbus_is_hotpluggable(dev->parent_bus); } return hotpluggable; } Thanks, Zhao