Re: qemu device model question

[George Dunlap <George.Dunlap@eu.citrix.com>]

On Tue, May 19, 2015 at 12:36 AM, Stefano Stabellini
<stefano.stabellini@eu.citrix.com> wrote:
> On Mon, 18 May 2015, Ian Campbell wrote:
>> On Mon, 2015-05-18 at 10:49 +0100, George Dunlap wrote:
>> > 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?
>>
>> FWIW xencommons starts a qemu for dom0 using:
>> $QEMU_XEN -xen-domid 0 -xen-attach -name dom0 -nographic -M xenpv -daemonize \
>>                 -monitor /dev/null -serial /dev/null -parallel /dev/null \
>>                 -pidfile $QEMU_PIDFILE
>>
>> I'd be surprised if this didn't also work for an arbitrary PV domain and
>> I assume it wouldn't be needed for an HVM one.
>
> It doesn't work for HVM guests because the QEMU device model also runs a
> qdisk backend instance. However it could work for PV/PVH guests.

...but it wouldn't be needed because the toolstack can talk to the
existing DM and tell it to start providing new PV block devices or
pvusb busses.

The discussion here is between starting a "dummy" qemu at start of day
if there are no qdisk or pvusb, or starting a qemu on-demand as it's
needed.  There's obviously no such choice for an HVM domain at the
moment, as it always needs qemu for a device model.

> FYI I am working on a set of patches to remove the handling of PV
> backends from QEMU device model. it requires both QEMU and libxl
> changes. Once they are in, you'll be able to do this for HVM guests too.

If you do this, then we'll need to solve the same problem for HVM
domains as Juergen is trying to solve for PV domains. :-)

But it sounds like it shouldn't be too hard.

 -George