Re: [Qemu-devel] drive_add vs. pci_add

[Anthony Liguori <anthony@codemonkey.ws>]

Paul Brook wrote:
> 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.
>   

Also, we probably want to separate the definition of storage from the 
act of connecting the storage via hotplug.  If I were going to add new 
monitor commands today, I may do something like:

# Create a new BlockDriverState with a symbolic name of 'Foo'
(qemu) storage_add Foo file=/path/to/image,snapshot=on

# Create a new pci device with a symbolic name of 'Bar'
(qemu) pci_create dev=virtio-blk,name=Bar

# Connect the storage backend Foo to the storage frontend Bar
(qemu) storage_connect Foo Bar

# Attach PCI device via hotplug
(qemu) pci_hotplug Bar addr=0.01.0

Beyond pci_hotplug, you could also introduce pci_insert/remove 
commands.  This could be used if starting a guest in the stopped state 
to insert a PCI card or in combination with -no-shutdown.

For attaching SCSI disks, you would probably introduce something like:

# Create a new scsi-disk named Bar
(qemu) scsi_create_lun target=0,lun=1,name=Bar

# Connect Foo storage backend to Bar storage frontend
(qemu) storage_connect Foo Bar

# Attach lun 'Bar' to existing SCSI controller named 'Controller'
(qemu) scsi_attach_lun Bar Controller

Regards,

Anthony Liguori