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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path()
Date: Tue, 15 Jun 2010 14:32:38 +0100
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To: Markus Armbruster <armbru@redhat.com>
Cc: "chrisw@redhat.com" <chrisw@redhat.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, Jan Kiszka <jan.kiszka@siemens.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Alex Williamson <alex.williamson@redhat.com>, "avi@redhat.com" <avi@redhat.com>, "kraxel@redhat.com" <kraxel@redhat.com>

> > > Every hotplug-capable bus must have a proper addressing scheme, I think
> > > this is a reasonable and achievable requirement. Then we don't need
> > > instance numbers for those buses.
> > 
> > What about USB?
> 
> USB has useful device addresses (physical ports).
> These aren't used by most of the higher-level protocols (logically USB is a
> flat bus structure), but the physically tree topology is still there if you
> look hard enough.

... and thinking about it a bit more, this adds an extra addressing 
possibility. 

USB devices have both physical and logical.  The physical addresses is which 
port the device is plugged into (including port addresses of any intermediate 
hubs). These never change, so should be the canonical address.

The logical address is set by the guest OS so may change (or be absent). Most 
software uses the logical address, so this is by far the most useful option 
for mapping from a guest device to a qemu device.

Paul