Re: [PATCH v2] qdev: Report an error for machine without HotplugHandler

[Akihiko Odaki]

On 2023/12/21 17:49, Markus Armbruster wrote:
> Akihiko Odaki <akihiko.odaki@daynix.com> writes:
>
> > On 2023/12/21 1:46, Zhao Liu wrote:
> > > 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 <armbru@redhat.com>
> > > > Subject: Re: [PATCH v2] qdev: Report an error for machine without
> > > >    HotplugHandler
> > > >
> > > > Akihiko Odaki <akihiko.odaki@daynix.com> writes:
> > > >
> > > > > On 2023/12/18 23:02, Markus Armbruster wrote:
> > > > > > Akihiko Odaki <akihiko.odaki@daynix.com> writes:
> > > > > >
> > > > > > > On 2023/12/11 15:51, Markus Armbruster wrote:
> > > > > > > > Akihiko Odaki <akihiko.odaki@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.odaki@daynix.com>
> > > > > > > >
> > > > > > > > Do you have a reproducer for the crash?
> > > > > > > >
> > > > > > > > > ---
> > > > > > > > > Changes in v2:
> > > > > > > > > - Fixed indention.
> > > > > > > > > - Link to v1: 
> > > > > > > > > 20231202-bus-v1-1-f7540e3a8d62@daynix.com">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.
> > > >
> > > > Three ideas to avoid the lying:
> > > >
> > > > 1. default hotpluggable to bus_type != NULL.
> >
> > I don't have an idea to achieve that. Currently bus_type is set after 
> > hotpluggable.
> >
> > > > 2. assert(dc->bus_type || !dc->hotpluggable) in a suitable spot.
> >
> > It results in abortion and doesn't improve the situation.
>
> Oh, it does!  The abort leads us to all the places where we currently
> lie (by having dc->hotpluggable = true when it isn't), so we can fix
> them.

I get your point. So we can put the assertion to the point that runs 
after device realization and let such devices break. It's quite 
disruptive but works.