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

[Paolo Bonzini]

Il 24/09/2014 13:48, Igor Mammedov ha scritto:
> usb-storage uses SCSI bus to provide underling storage
> (i.e. scsi-disk) and it's hotpluggable. In addition
> during usb-storage hotplug bus_add_child() won't allow
> to add scsi-disk to SCSI bus owned by usb-storage
> if bus is not hotpluggable.
> 
> By default SCSI bus was alway hotpluggable even if
> an owner didn't support hotplug. It will change
> once BusState.allow_hotplug field is dropped and
> SCSI bus will be only hotpluggable only if bus owner
> implements HOTPLUG_HANDLER interface.
> 
> So make usb-storage implement HOTPLUG_HANDLER interface
> to keep its SCSI bus hotpluggable.

Correct.  usb-bot unfortunately is not hot-pluggable because we would
plug it with no SCSI LUN inside.  The guest would not be able to access
it.  We would have to delay the presentation of the usb-bot device until
after the LUN is there.  Perhaps some kind of transaction mechanism?

However, usb-storage builds the outer USB device and the inner SCSI
device at the same time, so it can be hot-plugged.

What you do for usb-storage should also be done for lsi, esp, usb-uas, etc.

Paolo