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From: Markus Armbruster <armbru@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 0/3] v4 Decouple block device removal from
	device removal
References: <20101102202338.GB3469@us.ibm.com>
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	<20101105141708.GG17667@redhat.com>
Date: Fri, 05 Nov 2010 17:01:49 +0100
In-Reply-To: <20101105141708.GG17667@redhat.com> (Michael S. Tsirkin's message
	of "Fri, 5 Nov 2010 16:17:08 +0200")
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, yamahata@valinux.co.jp, qemu-devel@nongnu.org, Anthony Liguori <aliguori@linux.vnet.ibm.com>, Ryan Harper <ryanh@us.ibm.com>, Stefan Hajnoczi <stefan.hajnoczi@uk.ibm.com>

"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?

>> 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.