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

From: Juergen Gross <jgross@suse.com>
To: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Chunyan Liu <cyliu@suse.com>,
	Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Subject: Re: [PATCH 3/3] libxl: add new pvusb backend "qusb" provided by qemu
Date: Tue, 29 Mar 2016 12:35:27 +0200	[thread overview]
Message-ID: <56FA5A6F.4050905@suse.com> (raw)
In-Reply-To: <CAFLBxZaJo17LCUQu7qxKb7cdrtwLSbdm9WucVODiFoGj3RNPLA@mail.gmail.com>

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.

Juergen

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