Re: [PATCH 3/3] libxl: add new pvusb backend "qusb" provided by qemu

[Juergen Gross <jgross@suse.com>]

On 29/03/16 12:35, Juergen Gross wrote:
> On 29/03/16 11:45, George Dunlap wrote:
>> On Fri, Mar 25, 2016 at 6:09 AM, Juergen Gross <jgross@suse.com> wrote:
>>> On 24/03/16 21:07, Wei Liu wrote:
>>>> On Fri, Mar 18, 2016 at 09:24:26AM +0100, Juergen Gross wrote:
>>>>> On 17/03/16 17:55, George Dunlap wrote:
>>>>>> On Thu, Mar 10, 2016 at 3:00 PM, Juergen Gross <jgross@suse.com> wrote:
>>>>>>> Add a new pvusb backend type "qusb" which is provided by qemu. It can
>>>>>>> be selected either by specifying the type directly in the configuration
>>>>>>> or it is selected automatically by libxl in case there is no "usbback"
>>>>>>> driver loaded.
>>>>>>>
>>>>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>>>>
>>>>>> Oh, last thing -- shouldn't we have a way for libxl to tell whether
>>>>>> qemu can provide a qusb backend before we offer to start it up?
>>>>>
>>>>> How? Is this possible for qdisk? Or for an emulated frame buffer?
>>>>>
>>>>
>>>> I don't think it is  possible at the moment. But I think you might be
>>>> able to do it either via qmp command or parsing qemu help string. The
>>>> latter functionality is implemented in Ian's device model deprivilege
>>>> patch series IIRC -- but it is not yet merged either.
>>>
>>> Won't help for qusb, as there is no qemu parameter or help text related
>>> to the backend. It would be possible to add a qmp command to print the
>>> supported backend types, of course.
>>
>> Well as I understand it, for qemu-upstream we can't assume what
>> version of qemu we'll end up getting: we need to be compatible with
>> recent releases of qemu which don't provide qusb.  Which I'm pretty
>> sure means failing gracefully (i.e., giving a sensible warning
>> message) if someone tries to add qusb when their qemu binary doesn't
>> support it.
>>
>> If you can tell from the qemu failure that qusb is at fault, that's OK
>> I think; but if not, I think you'll have to add some way to query
>> whether qusb is supported by the qemu binary before starting it.
> 
> So we would want a qemu command line option (e.g. -xen-help) printing
> the supported pv backends. I think this should be easy to setup.

Actually there is a solution which is _really_ simple: before setting
it's state to "running" in xenstore, qemu could write the supported
backend types to xenstore, too. I'd add a xenstore directory

/local/domain/<backend-domid>/device-model/<domid>/backends

containing a directory for each supported backend. By using a directory
for each backend it would even be possible to indicate supported
features and/or parameters in the future.

"qusb" backend would be regarded to be supported only if
/local/domain/<backend-domid>/device-model/<domid>/backends/qusb
is existing.

"qdisk" would be regarded to be supported if either:
- /local/domain/<backend-domid>/device-model/<domid>/backends/qdisk
  is existing or:
- /local/domain/<backend-domid>/device-model/<domid>/backends
  is not existing.


Juergen

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