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

[Wei Liu <Wei.Liu2@xxxxxxxxxx>]

On Wed, 2013-01-02 at 16:53 +0000, Ian Campbell wrote:
> 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.
> 

Should be "the year before last year". Tt is 2013 now. ;-)

HVM should work. But PV transport layer was prototype and the "bus"
implementation was not elegant.

> 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.
> 

I happen to know a Sheepdog committer. I can ask him about the
portability of Sheepdog. Probably we can make this a GSoC project this
year?


Wei.


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