Re: [Qemu-devel] [libvirt] [PATCH 3/5] qemu: add usb-bot support from disks points of view

Re: [Qemu-devel] [libvirt] [PATCH 3/5] qemu: add usb-bot support from disks points of view

[Daniel P. Berrange]

On Mon, Sep 02, 2013 at 05:38:42PM +0800, Guannan Ren wrote:
> usb-bot only supports 16 luns(0~15) and they must be contiguous,
> (using lun 0 and 2 without 1 doesn't work). In this case qemu
> doesn't throw an error, we can not find the lun 2 in guests. So
> Adding a checking function in libvirt to prevent from this case.

Hmm, this seems like a problematic restriction.

How does this work if we start off a guest with 3 disks
attached to the usb-bot SCSI controller. Then hot-unplug
the 2nd disk.

Next time we need will need to start with with LUNs 0
and 2 populated only, otherwise we have ABI change upon
migrate or save/restore.

I think this restriction about contiguous luns must be
removed if this device is to be usable with multiple luns.

Daniel
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Re: [Qemu-devel] [libvirt] [PATCH 3/5] qemu: add usb-bot support from disks points of view

[Guannan Ren]

On 09/02/2013 08:57 PM, Daniel P. Berrange wrote:
> On Mon, Sep 02, 2013 at 05:38:42PM +0800, Guannan Ren wrote:
>> usb-bot only supports 16 luns(0~15) and they must be contiguous,
>> (using lun 0 and 2 without 1 doesn't work). In this case qemu
>> doesn't throw an error, we can not find the lun 2 in guests. So
>> Adding a checking function in libvirt to prevent from this case.
> Hmm, this seems like a problematic restriction.
>
> How does this work if we start off a guest with 3 disks
> attached to the usb-bot SCSI controller. Then hot-unplug
> the 2nd disk.

Currently, the qemu will throw out an error when I hot-unplug any of 
disks attached to usb-bot controller.

{ "execute": "device_del", "arguments": { "id": "scsi1-0-0-1"}}
{"error": {"class": "GenericError", "desc": "Bus 'scsi1.0' does not 
support hotplugging"}}

Libvirt will report an error:
# virsh detach-disk rhel64qcow2 sdd --current
error: Failed to detach disk
error: internal error: unable to execute QEMU command 'device_del': Bus 
'scsi1.0' does not support hotplugging



> Next time we need will need to start with with LUNs 0
> and 2 populated only, otherwise we have ABI change upon
> migrate or save/restore.
>
> I think this restriction about contiguous luns must be
> removed if this device is to be usable with multiple luns.
>
> Daniel

Re: [Qemu-devel] [libvirt] [PATCH 3/5] qemu: add usb-bot support from disks points of view

[Gerd Hoffmann]

On Mo, 2013-09-02 at 13:57 +0100, Daniel P. Berrange wrote:
> On Mon, Sep 02, 2013 at 05:38:42PM +0800, Guannan Ren wrote:
> > usb-bot only supports 16 luns(0~15) and they must be contiguous,
> > (using lun 0 and 2 without 1 doesn't work). In this case qemu
> > doesn't throw an error, we can not find the lun 2 in guests. So
> > Adding a checking function in libvirt to prevent from this case.
> 
> Hmm, this seems like a problematic restriction.

It's how the hardware works.

> How does this work if we start off a guest with 3 disks
> attached to the usb-bot SCSI controller. Then hot-unplug
> the 2nd disk.

You can't hotplug individual luns anyway.

cheers,
  Gerd

Re: [Qemu-devel] [libvirt] [PATCH 3/5] qemu: add usb-bot support from disks points of view

[Daniel P. Berrange]

On Tue, Sep 03, 2013 at 09:51:52AM +0200, Gerd Hoffmann wrote:
> On Mo, 2013-09-02 at 13:57 +0100, Daniel P. Berrange wrote:
> > On Mon, Sep 02, 2013 at 05:38:42PM +0800, Guannan Ren wrote:
> > > usb-bot only supports 16 luns(0~15) and they must be contiguous,
> > > (using lun 0 and 2 without 1 doesn't work). In this case qemu
> > > doesn't throw an error, we can not find the lun 2 in guests. So
> > > Adding a checking function in libvirt to prevent from this case.
> > 
> > Hmm, this seems like a problematic restriction.
> 
> It's how the hardware works.
> 
> > How does this work if we start off a guest with 3 disks
> > attached to the usb-bot SCSI controller. Then hot-unplug
> > the 2nd disk.
> 
> You can't hotplug individual luns anyway.

How does hotplug/unplug work in the context of usb-bot ?

AFAIK we need to be able to run

  device_add usb_bot
  drive_add file...
  device_add scsi-hd

And the reverse, to unplug it, if we're to have feature parity with
usb-storage.


Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|

Re: [Qemu-devel] [libvirt] [PATCH 3/5] qemu: add usb-bot support from disks points of view

[Gerd Hoffmann]

  Hi,

> > > How does this work if we start off a guest with 3 disks
> > > attached to the usb-bot SCSI controller. Then hot-unplug
> > > the 2nd disk.
> > 
> > You can't hotplug individual luns anyway.
> 
> How does hotplug/unplug work in the context of usb-bot ?
> 
> AFAIK we need to be able to run
> 
>   device_add usb_bot
>   drive_add file...
>   device_add scsi-hd
> 
> And the reverse, to unplug it, if we're to have feature parity with
> usb-storage.

Hot-unplug is easy.  You can remove the usb-bot device which will also
remove all child devices.

Hot-plug doesn't work at the moment, and I don't see any obvious way to
fix that properly :-(

We need some way to hotplug a *group* of devices (usb-bot + all
children) as usb-bot itself is hotpluggable but the individual scsi
devices connected to it are not.

I could allow hotplug on usb-bot as workaround, then you can do

  stop
  device_add usb_bot
  device_add scsi-{hd,cd,whatever}
  cont

but that would be more a gross hack than a solution ...

cheers,
  Gerd