Re: [Xen-devel] qemu device model question

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2015-05-13 at 14:06 +0100, Wei Liu wrote:
> 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
> though.
> 
> > 
> > > 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.

I didn't think we were talking about emulators, just pv backend (so
qdisk, qpvusb etc), those all have existing xenstore protocols based on
the backend dirs, all which would be needed would be a way to tell qemu
which backends a given process should take care of.

Ian.



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