Re: [Xen-devel] [PATCH v2 3/3] libxl: add domain config parameter to force start of qemu

[Juergen Gross <jgross@xxxxxxxx>]

On 29/03/16 16:27, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 29, 2016 at 06:44:32AM +0200, Juergen Gross wrote:
>> On 28/03/16 16:50, Konrad Rzeszutek Wilk wrote:
>>> On Tue, Mar 22, 2016 at 08:29:23AM +0100, Juergen Gross wrote:
>>>> Today the device model (qemu) is started for a pv domain only in case
>>>> a device requiring qemu is specified in the domain configuration
>>>> (qdisk, vfb, channel). If there is no such device the device model
>>>> isn't started and hence it is possible to add such a device to the
>>>> domain later.
>>>>
>>>> Add a domain configuration parameter to specify the device model is
>>>> to be started in any case. This will enable adding devices with a
>>>> qemu based backend later.
>>>
>>> .. s/devices/PV devices/
>>>
>>> As surely PV guests can't use emulated devices. Or could they? That
>>> would be quite interesting in a not kind of fun way.
>>
>> Would require some work.
>>
>>>
>>>>
>>>> While the optimal solution would be to start the device model
>>>> automatically when needed this would require some major rework of
>>>> libxl at multiple places.
>>>
>>> But you are not using this now (late start of QEMU), so why this patch?
>>
>> This was the reason for the patch: start the device model early in order
>> to have it running in case it would be needed later.
> 
> 
> OK. However I am missing something here. My understanding is that the
> QEMU Xen PV USB backend is not in the code-base. So why do this if QEMU
> won't even support this?
> 
> Would it make more sense to wait until QEMU has this support, then
> have this patch, or work on making libxl be able to start QEMU later
> (during the time QEMU gains the XEn PV USB backend)?

The qemu patches are sent already. I hope they are being accepted
rather soon.

I'm planning to add support for starting qemu as needed when 4.7 is
out.


Juergen

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