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Date: Tue, 24 Aug 2010 12:45:19 +0200
From: Alexander Graf <agraf@suse.de>
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Subject: Re: [Qemu-devel] [PATCH 4/5] Add generic drive hotplugging
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>, Luiz Capitulino <lcapitulino@redhat.com>, qemu-devel List <qemu-devel@nongnu.org>, Aurelien Jarno <aurelien@aurel32.net>, Gerd Hoffmann <kraxel@redhat.com>

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 :).

> 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?


Alex