Re: [Qemu-devel] [PATCH 26/30] usb-storage: make its storage SCSI bus hotpluggable explicitly

[Gerd Hoffmann]

On Mi, 2014-09-24 at 11:48 +0000, Igor Mammedov wrote:
> usb-storage uses SCSI bus to provide underling storage
> (i.e. scsi-disk) and it's hotpluggable.

No.  usb-storage itself (the scsi hba) is hotpluggable, but the scsi
devices connected are not.

On the qemu side it isn't a problem, but the protocol has constrains.
First there is no way to signal devices coming and going, so the guest
wouldn't see them until reboot (or manual driver reload).  Second for
multi-lun devices only the number of luns is reported and devices are
expected to have luns 0 .. $count-1.  So with three devices connected
and the second being hot-unplugged you end up with an illegal
configuration.

Therefore device initialization (IIRC) goes like this today:

  (1) add hba (i.e. usb-storage).
  (2) add scsi device.
  (3) turn off hotplug on the scsi bus.
  (4) make the whole thing visible to the guest (usb_attach_device).

cheers,
  Gerd