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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Fri, Mar 25, 2016 at 6:09 AM, Juergen Gross <jgross@xxxxxxxx> 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@xxxxxxxx> 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@xxxxxxxx>
>>>>
>>>> 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.

 -George

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