Re: [Qemu-devel] [RFC PATCH] Fix hotplug/hotunplug issue about virtio 1.0 devices

[Lin Ma]

在 2015年07月20日 23:36, Michael S. Tsirkin 写道:
> On Mon, Jul 20, 2015 at 11:14:58PM +0800, Lin Ma wrote:
> > Hi Michael and Gerd,
> >
> > I found an hotplug/hotunplug issue about virtio 1.0 devices and trying to
> > fix it.
> > The bug description is:
> > (qemu) device_add virtio-gpu-pci,id=gpu0
> > (qemu) device_del gpu0
> At this point, you must wait for guest to ack device removal.
After removing the device from qmp, The DEVICE_DELETED async event was 
shown.
So the guest already acked, Then add a device with same id, It always 
reports 'Duplicate ID ...'.
The device was completely removed from guest, But still in object list 
of qemu.
> > (qemu) device_add virtio-gpu-pci,id=gpu0
> > Duplicate ID 'gpu0' for device
> > Try "help device_add" for more information
> >
> > My fix looks like this:
> > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > index 283401a..098fc83 100644
> > --- a/hw/virtio/virtio-pci.c
> > +++ b/hw/virtio/virtio-pci.c
> > @@ -1413,6 +1413,12 @@ static void
> > virtio_pci_modern_region_map(VirtIOPCIProxy *proxy,
> >       virtio_pci_add_mem_cap(proxy, cap);
> >   }
> >
> > +static void virtio_pci_modern_region_unmap(VirtIOPCIProxy *proxy,
> > +                                           VirtIOPCIRegion *region)
> > +{
> > +    memory_region_del_subregion(&proxy->modern_bar, &region->mr);
> > +}
> > +
> >   /* This is called by virtio-bus just after the device is plugged. */
> >   static void virtio_pci_device_plugged(DeviceState *d, Error **errp)
> >   {
> > @@ -1584,6 +1590,16 @@ static void virtio_pci_exit(PCIDevice *pci_dev)
> >
> >       msix_uninit_exclusive_bar(pci_dev);
> >       address_space_destroy(&proxy->modern_as);
> > +    object_unparent(OBJECT(&proxy->modern_cfg));
> > +
> > +    bool modern = !(proxy->flags & VIRTIO_PCI_FLAG_DISABLE_MODERN);
> > +    if (modern) {
> > +        virtio_pci_modern_region_unmap(proxy, &proxy->common);
> > +        virtio_pci_modern_region_unmap(proxy, &proxy->isr);
> > +        virtio_pci_modern_region_unmap(proxy, &proxy->device);
> > +        virtio_pci_modern_region_unmap(proxy, &proxy->notify);
> > +    }
> > +    memory_region_unref(&proxy->modern_bar);
> >   }
> >
> >   static void virtio_pci_reset(DeviceState *qdev)
> >
> >
> >
> > But after applying the fix, I got the following errors:
> > (qemu) device_add virtio-gpu-pci,id=gpu0
> > (qemu) device_del gpu0
> > (qemu) **
> > ERROR:qom/object.c:825:object_unref: assertion failed: (obj->ref > 0)
> > ......
> >
> >
> > I think because of the grace period of rcu, the function
> > memory_region_unref(as->root) in do_address_space_destroy isn't performed
> > immediately,
> > The do_address_space_destroy is always performed after virtio_pci_exit, That
> > caused 'assertion failed: (obj->ref > 0)'.
> > I have no idea whether my guess is correct or in correct, and don't know how
> > to avoid this assertion failure in this situation.
> > Does my fix make sense ? May I have your ideas or suggestions?
> >
> > Thanks!
> > Lin