Re: [PATCH V3 4/6] xl: add pvusb commands

[Juergen Gross <jgross@suse.com>]

On 05/20/2015 04:41 PM, George Dunlap wrote:
> On Wed, May 20, 2015 at 3:33 PM, Juergen Gross <jgross@suse.com> wrote:
>> On 05/20/2015 04:20 PM, George Dunlap wrote:
>>>
>>> On Mon, Apr 20, 2015 at 9:12 AM, Juergen Gross <jgross@suse.com> wrote:
>>>>
>>>> On 04/19/2015 05:50 AM, Chunyan Liu wrote:
>>>>>
>>>>>
>>>>> Add pvusb commands: usb-ctrl-attach, usb-ctrl-detach, usb-list,
>>>>> usb-attach and usb-detach.
>>>>>
>>>>> To attach a usb device to guest through pvusb, one could follow
>>>>> following example:
>>>>>
>>>>>     #xl usb-ctrl-attach test_vm version=1 num_ports=8
>>>>>
>>>>>     #xl usb-list test_vm
>>>>>     will show the usb controllers and port usage under the domain.
>>>>>
>>>>>     #xl usb-attach test_vm 1.6
>>>>>     will find the first usable controller:port, and attach usb
>>>>>     device whose bus address is 1.6 (busnum is 1, devnum is 6)
>>>>>     to it. One could also specify which <controller> and which <port>.
>>>>>
>>>>>     #xl usb-detach test_vm 1.6
>>>>>
>>>>>     #xl usb-ctrl-detach test_vm dev_id
>>>>>     will destroy the controller with specified dev_id. Dev_id
>>>>>     can be traced in usb-list info.
>>>>>
>>>>> Signed-off-by: Chunyan Liu <cyliu@suse.com>
>>>>> Signed-off-by: Simon Cao <caobosimon@gmail.com>
>>>>> ---
>>>>> Changes to v2:
>>>>>      * use bus.addr as user interface instead of busid in
>>>>> usb-attach|detach
>>>>>      * remove usb-assignable-list interface
>>>>
>>>>
>>>>
>>>> Why? While lsusb in combination with xl usb-list for each domain will
>>>> give the same information, having to iterate through all domains can be
>>>> quite annoying.
>>>>
>>>> An alternative would be to accept omitting the domain for xl usb-list
>>>> and list all domains with assigned usb devices in this case.
>>>
>>>
>>> I don't understand what information it is that you want.  Do you want
>>> a list of devices *not already assigned* to domains?
>>
>>
>> Yes.
>
> ...and why do you need that, instead of just remembering what you'd
> assigned to whom?
>
> We don't really have the equivalent for pci either.  That is, if a
> device shows up in "lspci" but not in "pci-assignable-list", that may
> be either because 1) I hasn't yet been assigned to pciback (and this
> is available to be assigned to a domain), or 2) because it's already
> been assigned to a domain.  Someone new coming to the system would
> need to check all VMs to see which devices hadn't yet been assigned.

So this is a problem of pci-assignable-list, which isn't present for
USB devices. Any USB device not already assigned to a VM would be listed
as before with "xm usb-assignable-list".

Additionally all systems support hotplug of USB devices - the list of
available USB-devices can change rather often. If you unplug e.g. a
memory stick which has been assigned to a VM and stick it in again it
might show up under a different address and has to be reassigned.
Remembering having it already assigned won't help in this case as much
as a simple command to list the devices ready for assignment.


Juergen