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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] drive_add vs. pci_add
Date: Thu, 25 Jun 2009 17:02:42 +0100
References: <4A431D3D.2040604@web.de> <4A439A15.5040707@redhat.com>
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To: qemu-devel@nongnu.org
Cc: Jan Kiszka <jan.kiszka@web.de>, Avi Kivity <avi@redhat.com>

On Thursday 25 June 2009, Avi Kivity wrote:
> On 06/25/2009 09:46 AM, Jan Kiszka wrote:
> > Hi,
> >
> > sorry, it's still early, but isn't the monitor command 'drive_add'
> > completely redundant to 'pci_add ... storage'? If yes, and drive_add is
> > only there for legacy users, I would mask its help from the monitor
> > interface to avoid confusion.
>
> What we really want is pci_add storage to add a storage controller, and
> drive_add to attach a drive to that controller.  I don't think that's
> what happens now.

Part of the problem is that we don't currently isolate configs for different 
pats of the device stack. There are several different layers at which hotplug 
can occur:

- Device - e.g. PCI hotplug. If done properly this shouldn't care whether 
you're adding a NIC, VGA, SCSI HBA, or whatever.
- Drive - Adding/removing drives to an existing HBA.
- Media - e.g. changing the contents of a CDROM drive.

Depending on the subsystem, hotplug may only be supported at some levels. For 
example harddrives don't support media hotplug, IDE+virtio do not support 
drive hotplug, and the sun4m ESP SCSI adapter does not support device 
hotplug[1].

Paul

[1] I've no idea if actual sun4m SBUS hardware supports hotplug, but It 
suspect it does not.