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Date: Wed, 8 Jan 2014 18:17:49 +0100
From: Hani Benhabiles <kroosec@gmail.com>
Message-ID: <20140108171749.GB3717@Inspiron-3521>
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Subject: Re: [Qemu-devel] [PATCH] qdev: Assign a default device ID when none
 is provided.
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To: Markus Armbruster <armbru@redhat.com>
Cc: stefanha@redhat.com, qemu-devel@nongnu.org, aliguori@amazon.com, pbonzini@redhat.com, imammedo@redhat.com, afaerber@suse.de

On Wed, Jan 08, 2014 at 08:36:06AM +0100, Markus Armbruster wrote:
> Hani Benhabiles <kroosec@gmail.com> writes:
> 
> > This would allow a user to be able to refer to the device when using commands
> > like device_del.
> >
> > Signed-off-by: Hani Benhabiles <kroosec@gmail.com>
> 
> No.
> 
> Device IDs belong to the user.  Any IDs the system picks automatically
> can collide with the user's IDs.
> 
> Management applications assume that they can pick any ID they want.
> Your patch can introduce ID collisions, and thus make existing
> configurations fail.
> 

How can it lead to ID collisions ?

For this reason, the loop in assign_device_name() specifically check that the ID
doesn't exist already and uses the next value if it does.

How would something like:
(qemu) device_add virtio-net-pci
==> ID: virtio-net-pci.0

Be more problematic than:
(qemu) device_add virtio-net-pci,id=virtio-net-pci.0

> If I remember correctly, a few legacy convenience options pick IDs for
> historical reasons.  If you use them, you need to be aware of the IDs
> they pick.  Management applications shouldn't use them.
> 
> We've discussed this a couple of times already, by the way.