Re: [Qemu-devel] [PATCH 4/5] Add generic drive hotplugging

[Alexander Graf <agraf@suse.de>]

Daniel P. Berrange wrote:
> On Tue, Aug 24, 2010 at 03:40:25PM +0200, Alexander Graf wrote:
>   
>> Daniel P. Berrange wrote:
>>     
>>> On Tue, Aug 24, 2010 at 12:45:19PM +0200, Alexander Graf wrote:
>>>       
>>>>> The key is that you should use  if=none for all cases. Here are two
>>>>> examples of how libvirt does it currently:
>>>>>
>>>>> VirtIO:
>>>>>
>>>>>   drive_add dummy file=/var/lib/libvirt/images/data.img,if=none,id=drive-virtio-disk1,format=raw
>>>>>   device_add virtio-blk-pci,bus=pci.0,addr=0x0,drive=drive-virtio-disk1,id=virtio-disk1'
>>>>>
>>>>> SCSI:
>>>>>
>>>>>   drive_add dummy file=/var/lib/libvirt/images/data.img,if=none,id=drive-scsi0-0-1,format=raw'
>>>>>   device_add scsi-disk,bus=scsi0.0,scsi-id=1,drive=drive-scsi0-0-1,id=scsi0-0-1
>>>>>
>>>>> The 'dummy' value there can be absolutely anything you want.
>>>>> It is totaly ignored when QEMU sees if=none in 2nd arg.
>>>>>   
>>>>>       
>>>>>           
>>>> I'd be all for removing the pci-hotplug.c version of drive_add then. But
>>>> I think the IF_SCSI option there is to append a drive to an existing
>>>> SCSI bus, no?
>>>>     
>>>>         
>>> Actually this SCSI example I give above is appending a drive to an existing
>>> bus (scsi0), in slot 1 (scsi-id=1).  To best of my knowledge there is no
>>> remaining use case that requires use of IF_SCSI, IF_IDE, etc. The IF_NONE
>>> approach can cope with all, modulo bugs that appear periodically with code
>>> that mistakenly checks for a particular IF_XXX constant.
>>>
>>> If you wanted to also create a new SCSI bus, before creating the drive on
>>> it, you'd need to run three commands in total:
>>>
>>>   device_add lsi,id=scsi0,bus=pci.0,addr=0x7
>>>   drive_add dummy file=/var/lib/libvirt/images/data.img,if=none,id=drive-scsi0-0-1,format=raw
>>>   device_add scsi-disk,bus=scsi0.0,scsi-id=1,drive=drive-scsi0-0-1,id=scsi0-0-1
>>>   
>>>       
>> Nice - so we can just deprecate if=!none?
>>     
>
> In theory yes, but its not nice to tell users to switch everything over to
> use if=none, if we're going to deprecate that too in the next release when
> blockdev appears. Might as well just deprecate entire of drive_add/-drive
> at once.
>   

I guess I still fail to see the reason for blockdev when we force
drive_add to if=none...


Alex