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

Ian,  Thanks!  Can you elaborate just a bit (see below):

Ian Campbell wrote:
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.

Knew most of that... though the page has been greatly expanded since the last time I looked at it. It's the gory details I'm looking for...

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.

Thanks again,

Miles Fidelman

In theory, there is no difference between theory and practice.
In practice, there is.   .... Yogi Berra

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