Re: [Qemu-devel] [PATCH 0/3] v4 Decouple block device removal from device removal

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Nov 08, 2010 at 12:39:01PM -0600, Ryan Harper wrote:
> * Michael S. Tsirkin <mst@redhat.com> [2010-11-08 10:57]:
> > On Mon, Nov 08, 2010 at 08:02:50AM -0600, Ryan Harper wrote:
> > > * Markus Armbruster <armbru@redhat.com> [2010-11-08 06:04]:
> > > > "Michael S. Tsirkin" <mst@redhat.com> writes:
> > > > 
> > > > > On Mon, Nov 08, 2010 at 11:32:01AM +0100, Markus Armbruster wrote:
> > 
> > Daniel, I'd like your input here: can you live with
> > device diappearing from info block and parsing
> > qdev tree info to figure out whether device is really gone?
> 
> AFAICT, libvirt doesn't look at or use info block at all.
> 
> I'd rather not have to add info block to libvirt; but currently I can't
> see how else we can determine if we should call drive_unplug if we do a
> device_del() and the guest removes it before we call drive_unplug().  
> 
> What happens is that the guest removes the device and when we call
> drive_unplug() it fails to find the target device (since it was deleted
> by the guest). Then we fail the PCiDelDisk and libvirt keeps the device
> config around even though the guest has finished removing it.

This needs drive_unplug to return an explicitly identifiable
'no such device' error code, which libvirt can catch and
ignore.  Making the call to drive_unplug conditional on a
check to query-block/query-qdev is really a bug, because it
has an designed in race condition which means you need to 
check for a 'no such device' error code regardless. So it
is better to just blindly call drive_unplug and handle the
non-fatal failure conditions every time - this ensures that
codepath gets exercised more frequently too :-)

Regards,
Daniel
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