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

[Juergen Gross <jgross@suse.com>]

On 05/20/2015 05:25 PM, George Dunlap wrote:
> On 05/20/2015 03:55 PM, Juergen Gross wrote:
>> 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.
>
> OK. :-)  Yes, I can see that having USB devices disappear and appear as
> a different bus:host would make such a command particularly useful.
>
> But then we have the problem that "assignable" means something different
> in each case.  In pci-assignable-list, it means "Devices which have been
> assigned to pciback but not yet been attached to a domain".  In your
> suggested command, it means "Devices which have not yet been assigned
> either to pvusbback or to a domain."

For USB devices there isn't such an action as "assign it to pvusbback".
You just assign a USB device to a domain. One command, no steps to
prepare that action (besides the possibility to define a USB controller
first).

> When I introduced pci-assignable-* I didn't realize that "assignable"
> was already in one of the xm commands with a different meaning.  I
> introduced it because until that point, neither xm nor xl had a way of
> attaching a device to pciback.
>
> So we basically have three options:
> 1. Keep both names the same
> 2. Rename usb-assignable-list to something else (usb-available-list?)
> 3. Rename pci-assignable-* to something else
>
> #1 would be the most backwards-compatible.  But I think it's really bad
> going forward, since you have two commands that look like they should do
> similar things that don't.

Actually they do. :-)


Juergen

>
> #2 and #3 would both solve the potential confusion issue.
>
> #2 would be the easiest on current xl users.  It's a bit annoying
> interface-wise going forward, since "assignable" is probably the best
> word for what Juergen wants; and it's not 100% backwards-compatible with
> the previous xm usb commands.
>
> #3 would give us probably the most consistent naming thing going
> forward, but would be a pretty major breakage for current xl users.
>
> I'm inclined to suggest #2 as the best balance between not disrupting
> current users and not confusing future users.
>
> Wei / Ian, any opinions?
>
>   -George
>
>