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

On 09/03/2013 08:06 AM, Gerd Hoffmann wrote:
>   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.

There is another case where hot-plug/unplug of a group of devices would
be useful - if you want to hotplug/unplug a pci passthrough device using
vfio *and* that device is a part of an iommu group that contains other
devices *and* you're okay with having those other devices assigned to
the guest as well *AND* you want to use "managed='yes'" (where libvirt
automatically unbinds the devices from the host driver and binds them to
vfio-pci before passing them off to the guest), then you need to be able
to plug and unplug all the devices in that iommu group in a single
operation, otherwise the plug fails (due to attempting to assign one
device in a group when the other devices haven't been bound to vfio-pci
or pci-stub). As it stands now you can only hot-plug/unplug with vfio if
a device is in an iommu group all by itself, or if you don't use
managed='yes', and instead deal with binding the devices to vfio-pci

If none of that makes sense, consider yourself lucky :-)

> 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
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