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

[Markus Armbruster]

"Michael S. Tsirkin" <address@hidden> 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?

>> 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.