Re: [libvirt] [Qemu-devel] qapi DEVICE_DELETED event issued *before* instance_finalize?!
On 05/09/2016 11:23, Markus Armbruster wrote:
> >
> > On the other hand, it is clearly documented that the DEVICE_DELETED
> > event is sent as soon as guest acknowledges completion of device
> > removal. So libvirt's buggy if we'd follow documentation strictly. But
> > then again, I don't see much information value in "guest has detached
> > device but qemu hasn't yet" event. Libvirt would ignore such event.
Unless I'm missing something, libvirt needs an event that signals "Guest
and QEMU are done with this device". Current DEVICE_DELETED isn't.
Can we imagine a use for current DEVICE_DELETED, i.e. "Guest is done,
but QEMU isn't"?
Would anything break if we changed semantics of DEVICE_DELETED to what
libvirt actually needs?
If the answers are "no" and "no", let's do it.
There is a subtle aspect of this. After the current DEVICE_DELETED, the
device id is not used any more. So technically you could have
device_add bar,id=foo
device_del foo
// something in QEMU prevents the device from going away?
// for example there is a storage issue that blocks completion
// of a read(), and bar is a storage device
device_add bar,id=foo
device_del foo
// which foo is being deleted? The old one or the new one?
event DEVICE_DELETED
DEVICE_DELETED does have a meaning: management cannot talk to the device
anymore in QMP once it is raised.
Technically what libvirt wants to know for VFIO is not whether the
device is gone; it's whether the device's _backend_ (the VFIO file
descriptor) is gone. The device backend could have been a separate QOM
object, but it isn't.
So perhaps we need a new event that is specific to VFIO?
Thanks,
Paolo