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

[Alexander Graf]

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?


Alex