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Re: [Xen-devel] Driver domains and hotplug scripts, redux

On Tue, 31 Jan 2012, Roger Pau Monné wrote:
> That's what I don't get. Booting the driver domain should be no
> problem, because you can also have a xenbackendd running in the Dom0
> to boot the driver domain (or maybe you want to use both Dom0 and
> another DomU as driver domains).
> What I don't get is what you do when you have to boot a PV DomU which
> root HDD is on the driver domain. Dom0 needs the kernel/initrd from
> the HDD (usually extracted using pygrub). Since the HDD is inside the
> driver domain, Dom0 doesn't have access to that image, so there's no
> way to extract the kernel/initrd from the Dom0. What I through is that
> the driver domain has to run pygrub, extract the kernel/initrd, and
> pass both files to the Dom0, but how can we pass those files? libvchan
> seems like the best option, but I would like to head others opinions
> about this.

As Tim said, you would use blkfront in dom0 to get a device you can open
with pygrub.

> Currently I have a mostly working xenbackendd implementation with
> libxl, that can handle vbd and vif interfaces, but I'm missing qdisk,
> I still have to look into the Qemu stuff to be able to launch a device
> model that only attaches a HDD.
In case of qdisk, you can use the same blkfront in dom0 trick described
above, or, if the backend and the frontend are both in dom0, you can use
QEMU's nbdserver to export the disk and nbd-client to setup a device
that pygrub can open.
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