Re: [Xen-devel] qemu device model question

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, May 13, 2015 at 10:30 AM, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> On Wed, 2015-05-13 at 10:54 +0200, Juergen Gross wrote:
>> Hi,
>>
>> while trying to build a pvusb backend in qemu I think I've found a
>> general issue in xl: qemu for pv-domains is started only at domain
>> creation and only if there is at least one backend in qemu required.
>>
>> If there is no qemu process started for the domain at creation time
>> it will be impossible to successfully add such a device later while
>> the domain is running.
>>
>> Are there any plans to remove that restriction? Or have I missed
>> some mechanism in xl to start qemu at a later time?
>
> I think it would be reasonable to have some way to indicate that pvusb
> support is desired even if there are no such devices on boot, and for
> libxl to start the necessary backend in that case.
>
> s/pvusb/whatever/

In the pvusb case, I think it's the plan to have an option to specify
a pvusb *bus*, even with no devices attached.

But we should probably solve the more general problem. :-)  We should
have a way to plug in disks / other devices supplied by qemu after the
VM is already running.

Specifying an "I might want something later" option is suboptimal from
a UI point of view: if the admin forgets to add it when it's needed,
she'll have to reboot the VM; if she forgets to remove it when it's
not needed, she'll have a useless process lying around, potentially
increasing the surface of attack unnecessarily.  Starting a qdisk
(qemu-pv?) process on-demand would automatically give you the most
efficient option without additional UI baggage.

Is there really no way to start up a qdisk process after the domain is
created?  The qdisk process doesn't actually need to do any emulation,
after all -- it's just acting as a backend, right?

 -George

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