[Engine-devel] Stable PCI/DEvice addresses

[Dan Kenigsberg]

On Thu, Dec 01, 2011 at 02:09:42PM -0500, Andrew Cathrow wrote:
> 
> ----- Original Message -----
> > From: "Doron Fediuck" <dfediuck at redhat.com>
> > To: "Dan Kenigsberg" <danken at redhat.com>
> > Cc: engine-devel at ovirt.org, "Igor Lvovsky" <ilvovsky at redhat.com>
> > Sent: Thursday, December 1, 2011 10:52:42 AM
> > Subject: Re: [Engine-devel] Stable PCI/DEvice addresses
> >
> > On Thursday 01 December 2011 16:11:07 Dan Kenigsberg wrote:
> > > On Thu, Dec 01, 2011 at 02:06:52PM +0200, Livnat Peer wrote:
> > > > Moving this back to list -
> > > >
> > > > On 12/01/2011 01:49 PM, Dan Kenigsberg wrote:
> > > > > On Thu, Dec 01, 2011 at 06:26:16AM -0500, Eli Mesika wrote:
> > > > >> Hi guys
> > > > >>
> > > > >> I need the xml/json format representing the VM installed
> > > > >> devices.
> > > > >> Livnat asked me to add it to my Wiki
> > > > >> http://www.ovirt.org/wiki/Features/Design/StableDeviceAddresses
> > > > >>
> > > > >> Please feel free to either send it to me or edit the VDSM
> > > > >> section adding this info.
> > > > >
> > > > > I think that it is wrong to document this in this point in
> > > > > time. The
> > > > > data is a blob, generated by libvirt, copied by Vdsm, and not
> > > > > expected
> > > > > to be editted by RHEV-M.
> > > > >
> > > > > If you REALLY want to know, it is simply libvirt's domain xml,
> > > > > which is
> > > > > well-documented in http://libvirt.org/formatdomain.html.
> > > > >
> > > > > Dan.
> > > > >
> > > >
> > > > Hi Dan,
> > > >
> > > > Since i suspect the next requirement on this would be for RHEVM
> > > > to parse
> > > > the "blob" and enable user to specify addresses i think the
> > > > content of
> > > > the "blob" should be discussed.
> > > >
> > > > Otherwise we'll have to support this "blob" format for the sake
> > > > of
> > > > backwards compatibility and not be able to set a reasonable API
> > > > between
> > > > the engine and VDSM.
> > >
> > > The requirement for 3.1 alowed me to define an opaque parameter,
> > > with
> > > which Vdsm uses the Engine to store the VM device addresses.
> > >
> > > We "secretly" opted for storing the libvirt domxml because it
> > > already
> > > contains addresses for just about anything, and would alow us to do
> > > even more evil things in the future (I'm thinking about specifying
> > > complete boot order, and other things which libvirt long supports,
> > > but
> > > Vdsm does not). Another reason was that this was a very simple
> > > thing
> > > to do.  The down side is that this "device-blob" is a bit bloated
> > > in
> > > size, and if you look deep into it, it has duplicate information on
> > > top
> > > of Vdsm's "create" verb.
> > >
> > > We should probably not include the <source> elements in the blob
> > > they
> > > are very verbose and uninteresting to RHEV-M.
> > >
> > > If stressed, Vdsm could report only the <devices> element. It could
> > > also
> > > convert it to json or yaml, compress and encrypt it - but I do not
> > > see
> > > the point of these transformations.
> > >
> > > Dan.
> > >
> > The direction this is taking is for Engine core to be able to parse
> > and
> > edit libvirt's domxml, while vdsm is agnostic (or partially agnostic)
> > to
> > the blob.
> >
> > Is this what we really need? want?
> 
> When we discuss the blob are we talking about just the address part - eg .
> <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
> 
> or something more?

This is exactly the question. Until today, the blob was just a blob, opaque to
the engine, and there was no discussion.

If the engine wants to be able to control and edit the device addresses, we must
agree on how to represent the address for each device type, and how to name each
device. The simplest solution is to tap on the good work of the libvirt chaps.
If we ever want to, it would allow the Engine to do the crazier stuff alluded by
Doron - but it does not force us to go down this path, though.

In my original design I imagined the blob to be a complete libvirt domxml. However,
I understand the benefits of a more specific "blob". For the feature at hand, it
is safe to limit this down to the <devices> element, with its <source> elements
stripped, and possibly converted to your data representation language du jour.

Dan.