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Re: [Xen-devel] qemu device model question

On Wed, May 13, 2015 at 10:30:40AM +0100, Ian Campbell 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/

libxl__need_xenpv_qemu comes to mind.

Also QEMU has a mode called attach mode. Never use it personally

> > Related to this problem is the question whether it would be desirable
> > to have e.g. qdisk and pvusb backends in the same process possibly
> > influencing each others throughput.
> Dissagregation of qemu's PV functionality would be nice, but I'm not
> sure how easy/hard it would be.

At least we would need to invent a new xenstore protocol to support
multiple emulator.


> Ian.
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