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Re: [Xen-users] where does qemu fit in?

On Wed, 2013-01-02 at 16:35 +0000, Miles Fidelman wrote:

> > The main use of QEMU in Xen is to provide emulation of the PC platform
> > (i.e. the random devices in a typical PC, like key board controller,
> > north/south bridge, PCI bus etc for fully virtualised (HVM) guests. PV
> > guests have no emulated devices and therefore QEMU is not use in this
> > capacity there.
> >
> > The other use is that qemu can optionally be used to provide certain PV
> > device backends. Most commonly these are provided by the dom0 or driver
> > domain kernel drivers but for example the "xenfb" (framebuffer PV
> > backend) lives in qemu. You can also (optionally) use the "qdisk"
> > backend for the PV disk backend, the other options are the in-kernel
> > blkback or blktap2. If you have any of these configured then you might
> > get a qemu process for a PV guest too.
> >
> Can you elaborate on this just a bit?  It sounds like one can use qemu 
> drivers from an HVM, to access the PV drivers.  But what about the other 
> way around?  I'm running PV guests, so normally qemu doesn't apply, 
> but... what I'm trying to figure out is whether there's some way to use 
> the qemu drivers to access the sheepdog cluster file system - which only 
> provides a qemu interface.

I'm not sure what you mean by "qemu drivers". Do you mean virtio?

virtio and the Xen PV drivers fill the same niche for KVM and Xen
respectively, but there is no concept of using virtio to access the Xen
PV drivers (that doesn't really make conceptual sense since they are the
same thing)

There was a summer of code project last year to implement virtio support
for Xen directly. I think it was working for HVM but I don't think the
PV version got completed and since the qemu folks weren't keen on a new
virtio "bus" it was never pursued further.

If you meant emulated devices rather than virtio then there is no way to
have a PV guest use an emulated device.

Looking (very briefly) at the sheepdog page it looks KVM specific to me,
I wouldn't expect it to Just Work on Xen -- someone would need to do a
porting job.


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