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Re: [Xen-devel] [PATCH 2/3] libxl: add domain config parameter to force start of qemu

On 18/03/16 08:11, Juergen Gross wrote:
> On 17/03/16 17:06, George Dunlap wrote:
>> On Thu, Mar 10, 2016 at 3:00 PM, Juergen Gross <jgross@xxxxxxxx> 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.
>>> 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>
>> So wait -- what happens now if you try to attach a disk with a qdisk
>> backend to a PV guest that didn't start with qemu running?
> It won't work (that was my test case for the patch).
>> I'd really like to see patch 3 get in, but I'm not really in favor of
>> this sort of a user-visible hack, particularly as we have to support
>> it in libxl more or less indefinitely.
> Hmm, really? We can add a smarter variant later which will start the
> device model in case it isn't started yet. Then the new config parameter
> could be just ignored.

Sure; but it will (probably) only actually be useful for one release
cycle (now 6 months), and then it will sit around cluttering up the
interface for years to come.  The situation wrt hotplug has been this
way for years now, and nobody has complained; I don't think an extra 6
months will be a big deal.

Ultimately it's the tools maintainers' call; I wouldn't argue against it
if one of them think it's a good idea.

> I didn't do the smart variant as I'm not sure I could set it up in time
> for 4.7. I'd be happy to do it with some assistance regarding the async
> framework of libxl I'm not at all familiar with.

I'm sure help could be arranged; but it might be difficult to do by 4.7.


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