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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 05, 2010 at 05:01:49PM +0100, Markus Armbruster wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Fri, Nov 05, 2010 at 02:27:49PM +0100, Markus Armbruster wrote:
> >> I'd be fine with any of these:
> >> 
> >> 1. A new command "device_disconnet ID" (or similar name) to disconnect
> >>    device ID from any host parts.  Nice touch: you don't have to know
> >>    about the device's host part(s) to disconnect it.  But it might be
> >>    more work than the other two.
> >> 
> >> 2. New commands netdev_disconnect, drive_disconnect (or similar names)
> >>    to disconnect a host part from a guest device.  Like (1), except you
> >>    have to point to the other end of the connection to cut it.
> >
> > I think it's cleaner not to introduce a concept of a disconnected
> > backend.
> 
> Backends start disconnected, so the concept already exists.
> 
> > One thing that we must be careful to explicitly disallow, is
> > reconnecting guest to another host backend. The reason being
> > that guest might rely on backend features and changing these
> > would break this.
> >
> > Given that, disconnecting without delete isn't helpful.
> 
> What about disconnect, hot plug new device, connect?

Exactly. I don't think we want to support this.
New device might not support all features that old one has.
Or it may have more features.

> >> 3. A new command "drive_del ID" similar to existing netdev_del.  This is
> >>    (2) fused with delete.  Conceptual wart: you can't disconnect and
> >>    keep the host part around.  Moreover, delete is slightly dangerous,
> >>    because it renders any guest device still using the host part
> >>    useless.
> >
> > I don't see how it's more dangerous than disconnecting.
> > If guest can't access the backend it might not exist
> > as far as guest is concerned.
> 
> If we keep disconnect and delete separate operations, we can make delete
> fail when still connected.  Typo insurance.
> 
> >> Do you need anything else from me to make progress?
> >
> > Let's go for 3. Need for 1/2 seems dubious, and it's much harder
> > to support.