Re: [Qemu-devel] [PATCH v2] virtio: Move memory_listener_unregister to .unrealize

[Paolo Bonzini <pbonzini@redhat.com>]

> On Tue, 05/16 14:44, Paolo Bonzini wrote:
> > On 16/05/2017 14:25, Fam Zheng wrote:
> > > You are right. Having had another look, I think it's because of this:
> > > VirtIODevice is an embeded member of VirtIOSCSIPCI therefore it is never
> > > "finalized" through QOM reference directly.  Am I right?
> > 
> > What I would expect is:
> > 
> > virtio_instance_init_common:
> > - create the VirtIODevice with refcount 1
> > - create a child property for the VirtIODevice (refcount is now 2)
> > - unref the VirtIODevice (refcount is again 1)
> > 
> > virtio_pci_realize:
> > - virtio_pci_bus_new creates the virtio bus
> > - the virtio bus is added as a child property
> > 
> > virtio_scsi_pci_realize:
> > - qdev_set_parent_bus links the device to the bus (bus and VirtIODevice
> > refcounts are now 3)
> > - the VirtIODevice is realized
> > 
> > ...
> > at hot-unplug time:
> > - the device is unrealized
> > - the bus is unparented (calling bus_unparent)
> >   - the device is unparented (calling device_unparent)
> >     - bus_remove_child is called (bus and VirtIODevice refcounts are now 1)
> >     - the VirtIODevice child property is deleted by object_unparent and
> > the VirtIODevice is finalized
> >   - the bus child property is deleted by object_unparent and the
> > VirtIODevice is finalized
> 
> Sorry I don't understand. From my debugging, VirtIODevice is not finalized,
> because it is embeded as VirtIOSCSIPCI.vdev.parent_obj.parent_obj, in
> non-zero offset.

It is correct that it's embedded.  But it is added as a child property,
and when the child property is deleted the refcount should go to zero and
the VirtIODevice should be deleted too.

The child property is deleted when bus_unparent calls object_unparent:

    while ((kid = QTAILQ_FIRST(&bus->children)) != NULL) {
        DeviceState *dev = kid->child;
        object_unparent(OBJECT(dev));
    }

and in turn bus_unparent is called by the VirtIOSCSIPCI's unparent
callback (device_unparent):

    while (dev->num_child_bus) {
        bus = QLIST_FIRST(&dev->child_bus);
        object_unparent(OBJECT(bus));
    }

Thanks,

Paolo


> 
> As I understand it, it's the VirtIOSCSIPCI instance that is being finalized,
> but
> not VirtIODevice. Because the object_unparent is actually called on the
> containing object:
> 
> Thread 4 "qemu-system-x86" hit Breakpoint 1, device_unparent
> (obj=0x559449824f40) at /stor/work/qemu/hw/core/qdev.c:1078
> 1078	    DeviceState *dev = DEVICE(obj);
> (gdb) bt
> #0  0x00005594455f11df in device_unparent (obj=0x559449824f40) at
> /stor/work/qemu/hw/core/qdev.c:1078
> #1  0x00005594457be582 in object_finalize_child_property (obj=0x559447f0d320,
> name=0x5594498306e0 "scsi1", opaque=0x559449824f40) at
> /stor/work/qemu/qom/object.c:1369
> #2  0x00005594457bc34a in object_property_del_child (obj=0x559447f0d320,
> child=0x559449824f40, errp=0x0) at /stor/work/qemu/qom/object.c:428
> #3  0x00005594457bc42a in object_unparent (obj=0x559449824f40) at
> /stor/work/qemu/qom/object.c:447
> #4  0x00005594455acf19 in acpi_pcihp_eject_slot (s=0x55944967c440, bsel=0,
> slots=16) at /stor/work/qemu/hw/acpi/pcihp.c:138
> #5  0x00005594455ad542 in pci_write (opaque=0x55944967c440, addr=8, data=16,
> size=4) at /stor/work/qemu/hw/acpi/pcihp.c:272
> #6  0x0000559445437667 in memory_region_write_accessor (mr=0x55944967d050,
> addr=8, value=0x7fdfa7569838, size=4, shift=0, mask=4294967295, attrs=...)
>     at /stor/work/qemu/memory.c:526
> #7  0x000055944543787f in access_with_adjusted_size (addr=8,
> value=0x7fdfa7569838, size=4, access_size_min=1, access_size_max=4, access=
>     0x55944543757d <memory_region_write_accessor>, mr=0x55944967d050,
>     attrs=...) at /stor/work/qemu/memory.c:592
> #8  0x0000559445439fdb in memory_region_dispatch_write (mr=0x55944967d050,
> addr=8, data=16, size=4, attrs=...) at /stor/work/qemu/memory.c:1319
> #9  0x00005594453dfa77 in address_space_write_continue (as=0x559445f1fce0
> <address_space_io>, addr=44552, attrs=..., buf=0x7fdfb77d2000 "\020", len=4,
> addr1=8, l=4, mr=0x55944967d050) at /stor/work/qemu/exec.c:2822
> #10 0x00005594453dfc31 in address_space_write (as=0x559445f1fce0
> <address_space_io>, addr=44552, attrs=..., buf=0x7fdfb77d2000 "\020", len=4)
> at /stor/work/qemu/exec.c:2879
> #11 0x00005594453dffbd in address_space_rw (as=0x559445f1fce0
> <address_space_io>, addr=44552, attrs=..., buf=0x7fdfb77d2000 "\020", len=4,
> is_write=true)
>     at /stor/work/qemu/exec.c:2981
> #12 0x0000559445433797 in kvm_handle_io (port=44552, attrs=...,
> data=0x7fdfb77d2000, direction=1, size=4, count=1) at
> /stor/work/qemu/kvm-all.c:1803
> #13 0x0000559445433e87 in kvm_cpu_exec (cpu=0x559447f0d560) at
> /stor/work/qemu/kvm-all.c:2032
> #14 0x0000559445419cc6 in qemu_kvm_cpu_thread_fn (arg=0x559447f0d560) at
> /stor/work/qemu/cpus.c:1118
> #15 0x00007fdfb56ba6ca in start_thread () at /lib64/libpthread.so.0
> #16 0x00007fdfb1382f7f in clone () at /lib64/libc.so.6
> (gdb) p *obj.class.type
> $1 = {name = 0x559447e57a20 "virtio-scsi-pci", class_size = 280,
> instance_size = 34688, class_init = 0x55944573573e
> <virtio_scsi_pci_class_init>, class_base_init = 0x0,
>   class_finalize = 0x0, class_data = 0x0, instance_init = 0x559445735837
>   <virtio_scsi_pci_instance_init>, instance_post_init = 0x0,
>   instance_finalize = 0x0,
>   abstract = false, parent = 0x559447e57a40 "virtio-pci", parent_type =
>   0x559447e57520, class = 0x559447e7add0, num_interfaces = 0, interfaces =
>   {{
>       typename = 0x0} <repeats 32 times>}}
> 
> Am I missing something?
> 
> Fam
>