Re: [PATCH 3/3] libxl: libxl__device_from_disk should retrieve backend from xenstore

[Wei Liu <wei.liu2@citrix.com>]

Sorry for the late reply.

On Thu, Feb 26, 2015 at 04:49:21PM -0700, Jim Fehlig wrote:
> Wei Liu wrote:
> > On Wed, Feb 11, 2015 at 10:18:18AM -0700, Jim Fehlig wrote:
> >   
> >> At minimum, libvirt will populate the pdev_path, vdev, backend, and
> >> format fields. If backend and format (which, in libvirt-speack
> >> correspond to the 'name' and 'type' attributes on the optional <driver>
> >> element) are not specified, they are set to LIBXL_DISK_BACKEND_UNKNOWN
> >> and LIBXL_DISK_FORMAT_RAW respectively.
> >>
> >>     
> >
> > Since libvirt has a tendency of specifying everything, how come there is
> > no "name" and "type" in <driver>?
> 
> The <driver> element is optional. From
> http://libvirt.org/formatdomain.html#elementsDisks
> 
> "|driver: |The optional driver element allows specifying further details
> related to the hypervisor driver used to provide the disk"
> 
> And when not specified, Ian C. recommended allowing libxl to pick
> suitable defaults
> 
> https://www.redhat.com/archives/libvir-list/2013-February/msg01126.html
> 
> >  Can we actually generate all the
> >   
> > fields needed when attaching a disk and store in libvirt's diskspec?
> 
> Yes, it was this way before the suggested change.
> 

I'm now of the opinion that we shouldn't change libxl__device_from_disk
to fill in specific information, otherwise it becomes a special case for
all the libxl__device_from_$foo  functions.

And from the information you provide above it seems to be easily fixable
on libvirt side -- you have all the information at hand when attaching
the disk, so why not just store it for reuse later?

Ian C, what do you think?

Wei.

> Regards,
> Jim