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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Sun, 2012-12-30 at 20:14 +0000, Miles Fidelman wrote:
> Hi Folks,
> 
> I've been looking a bit at various cluster file systems, and I keep 
> coming back to their qemu interfaces.  But... I really don't understand 
> where qemu fits into xen.
> 
> Can anybody explain where and how qemu is utilized within xen - for 
> paravirtualized VMs vs. VMs that use hardware virtualization extensions 
> vs. paravirtualized VMs that take advantage of some hardware 
> virtualization extensions?

You might find that http://wiki.xen.org/wiki/Xen_Overview answers some
of your question.

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.

Ian.



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