Thanks Mark,
Yes, this generally works (I've tried block devices on xen-3.2-testing).
Some additional details:
1) If you build two different kernels dom0 and domU don't forget to
enable XEN_BACKEND, XEN_BLKDEV_BACKEND, XEN_NETDEV_BACKEND in your
domU config.
2) To connect backend you need a domU with a Linux system with working
hotplug scripts (I guess BSD and other systems use something else).
Hotplug scripts in turn depend on working xenstore-write,
xenstore-read, xenstore-..., etc. tools.
Some bugs/unclear features:
1) If you ask to import the same device, but from different domains,
like /dev/loop0 below:
disk = ['phy:/dev/loop0,sda1,w',
'phy:/dev/loop0,sda2,w,block-backend-domain']
Domain is not created with an error:
Error: The file "/dev/loop0" is already used
2) If you export a block device from dom0 through a domU to the second
domU, e.g.
/dev/loop0 ----> /dev/sda2 ----> /dev/sda2
dom0 domU(1) domU(2)
device is not accessible in domU(2) with an error
Buffer I/O error on device sda2, logical block 25840
Xen complains:
(XEN) grant_table.c:360:d0 Could not pin grant frame 99551
Anton
On Fri, Jul 18, 2008 at 12:30:48AM +0100, Mark Williamson wrote:
> On Thursday 17 July 2008, Anton Burtsev wrote:
> > Hi,
> >
> > Is there a way to configure/hack Xen to run backend devices in
> > a non-dom0 domain (in domU). Like, for example connect two domU
> > domains with a block device.
>
> Yes, I think this should Just Work with the right configuration parameters...
>
> Set:
> blkif = 'yes'
> or
> netif = 'yes'
>
> As appropriate in your domain config file for the "backend" domU. Then in
> the
> frontend domU's config file you can specify something like:
>
> disks = [ 'phy:hda1,hda1,w,BACK' ]
>
> substituting the domid of the backend domain for BACK (its name might work,
> possibly, I'm not sure). Specifying file: or tap: disks in the backend domU
> to work in this way might be a bit of a fiddle (if it's possible) but phy:
> should work just fine.
>
> or you can do:
>
> vif
> =
> "type=TYPE,mac=MAC,bridge=BRIDGE,ip=IPADDR,script=SCRIPT,backend=DOM,vifname=NAME"
>
> again, specifying the domid (or possibly the name) of the backend domain.
>
> If you start the backend and then the frontend from these config files they
> should be able to connect do virtual device IO.
>
> Cheers,
> Mark
>
>
> --
> Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)
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