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

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Aug 24, 2010 at 03:46:14PM +0200, Alexander Graf wrote:
> 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...

Markus can no doubt explain better than me, but off the top of my head
 
 - 'drive' has guest properties that should be against the device
   not the drive which is for host properties (eg serial=, if=)
 - 'file' is mangled to include protocol/format which means that 
   it can't be unambigously parsed. (eg filenames containing :)

Fixing those, particularly the latter, would breaks back-compat so we
really need a new arg with sensible definition. This is what blockdev
is intended todo (as well as a internal code cleanup)

Regards,
Daniel
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