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Re: [Xen-devel] [PATCH v2] libxl: fix migration of PV and PVH domUs with and without qemu

On Thu, May 09, 2019 at 04:29:56PM +0200, Olaf Hering wrote:
> Am Thu, 9 May 2019 15:56:21 +0200
> schrieb Olaf Hering <olaf@xxxxxxxxx>:
> > Am Fri, 3 May 2019 13:04:11 +0200
> > schrieb Roger Pau Monné <roger.pau@xxxxxxxxxx>:
> > 
> > > I think the above call is wrong, libxl__need_xenpv_qemu expects to get
> > > the domid of the toolstack domain (ie: the domain running this code),
> > > not the domain being created.  
> > 
> > So, how do I actually test such setups? It seems a driver domain is
> > required. According to xl-disk-configuration(5) I may need to specify
> > backend=$domU. Is there some guide how to configure such thing?

Hm, I'm afraid I'm not able to find anything, I could swear I've
added something to the wiki when this was implemented.

Anyway, you need to create a domU and add driver_domain=1 to the
domain xl.cfg. Inside this domU you need `xl devd` running, there's an
init script for it. Note you will have to install the Xen toolstack
inside this domU. That would be the domU serving the backends.

Then you create another domU that would have a disk line with
backend=<backend domU> and have the disk image available in the
backend domU.

That should be enough to get it working as a test.

> While my question still stands, I wonder if the statement regarding
> libxl__need_xenpv_qemu is correct.

Hm, TBH I'm not sure libxl__need_xenpv_qemu is still used by driver
domains (devd). AFAICT the decision whether to use qdisk or not should
be set on the disk configuration line manually, and then the driver
domain just launches a qemu instance when required (ie: when using

> Are you saying the current users of libxl__need_xenpv_qemu 
> (libxl__dm_check_start,
> spawn_stub_launch_dm and domcreate_launch_dm) do not only run in dom0, but
> also somewhere else?

libxl__spawn_qdisk_backend can indeed be called from driver domains
(!= dom0). See add_device which gets called by backend_watch_callback
from the flow of `xl devd`.

Thanks, Roger.

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