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

[Daniel P. Berrange]

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:
> >   
> >> 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 :-)
> >   
> 
> And moving it out of the pci specific file. Yeah :). Basically my
> proposal was to take the patches as is and phase out the pci-hotplug.c
> version.
> 
> >   
> >>> 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.

Regards,
Daniel
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