Re: [Xen-devel] [PATCH v3 5/5] libxl: add domain config parameter to force start of qemu

[Juergen Gross <jgross@xxxxxxxx>]

On 24/03/16 21:35, Wei Liu wrote:
> On Thu, Mar 24, 2016 at 08:28:45PM +0000, Wei Liu wrote:
>> On Wed, Mar 23, 2016 at 01:24:07PM +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 impossible 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.
>>>
>>> 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.
>>>
>>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>>
>> I can't say I like this.
>>
>> Libxl can be made smart enough to detect whether starting a device model
>> is required (as you already did in previous patch) then start qemu on
>> demand.
>>
>> There is a xen-attach mode in qemu, and libxl already has machinery for
>> starting qemu. I suspect what you need to do is refactoring existing
>> code and expose the right parameter for qemu. Then you wouldn't need
>> this user visible option at all. You would also save the resources for
>> always having an idle qemu in toolstack domain.
>>
>> Let me know if this is to vague.
>>
> 
> Actually I don't think this is your problem. I would consider this a bug
> in libxl for not starting qemu on demand. I can take a stab at it, but
> that will need to wait until next release.
> 
> I think the functionality you care about  can live without this patch,
> but I'm not sure if it would become too cumbersome to use.

The "normal" block backend is qdisk these days. So normally the device
model will be running. And in many cases a pvusb device will be assigned
via the xl config of the domain which will start the device model
automatically.

I just wanted to make clear there is a potential problem and presented
a solution for this which I could implement in time for 4.7.


Juergen


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