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

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

On Tue, Aug 24, 2010 at 12:45:19PM +0200, Alexander Graf wrote:
> Daniel P. Berrange wrote:
> > On Tue, Aug 24, 2010 at 12:02:30AM +0200, Alexander Graf wrote:
> >   
> >> The monitor command for hotplugging is in i386 specific code. This is just
> >> plain wrong, as S390 just learned how to do hotplugging too and needs to
> >> get drives for that.
> >>
> >> So let's add a generic copy to generic code that handles drive_add in a
> >> way that doesn't have pci dependencies.
> >>
> >> I'm not fully happy with the patch as is. IMHO there should only be a
> >> single target agnostic drive_hot_add function available. How we could
> >> potentially fit IF_SCSI in there I don't know though.
> >>     
> >
> > I'm not sure that this patch is actually neccessary. Via a undocumented,
> > sick, dirty hack, you can already use the current drive_add command
> > without a PCI address, for both virtio + scsi. In fact not using the
> > PCI address with drive_add is the preferred approach in the new qdev
> > world even on x86
> >   
> 
> It is certainly necessary since the current code is in a big fat #if
> defined(TARGET_I386) block :).

True, true, killing the #ifdef is needed :-)

> > 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

Regards,
Daniel
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