[Qemu-devel] [RFC] Machine description as data

[Hollis Blanchard]

On Thu, 2009-02-12 at 11:26 +0100, Markus Armbruster wrote:
>  David Gibson <dwg at au1.ibm.com> writes:
> 
> > On Wed, Feb 11, 2009 at 12:50:28PM -0600, Hollis Blanchard wrote:
> >> On Wed, 2009-02-11 at 16:40 +0100, Markus Armbruster wrote:
> > [snip]
> >> > I briefly examined the DT source format and the tree structure it
> >> > describes for the purpose of QEMU configuration.  I decided
> against
> >> > using it in my prototype because I found it awfully low-level and
> >> > verbose for that purpose (I'm sure it serves the purpose it was
> designed
> >> > for just fine).  Issues include:
> >> > 
> >> > * Since the DT is designed for booting kernels, not configuring
> QEMU,
> >> >   there's information that has no place in QEMU configuration,
> and
> >> >   required QEMU configuration isn't there.
> >> 
> >> What's needed is a "binding" in IEEE1275-speak: a document that
> >> describes qemu-specific nodes/properties and how they are to be
> >> interpreted.
> >> 
> >> As an example, you could require that block devices contain
> properties
> >> named "qemu,path", "qemu,backend", etc.
> >
> > Yes, it shouldn't be hard to annotate an IEEE1275 style tree with
> > extra information for qemu's use.
> 
> I don't feel up to that task, because I'm not really familiar with
> IEEE1275.  Could you help out?

I'm not really a "language lawyer" for device trees, but I can help.

FWIW, I was imagining (from a PowerPC point of view) that a strict
subset of the device tree interpreted by qemu would be passed into the
guest. In other words, once qemu is done with it, it would strip every
property prefixed with "qemu," and copy the result into guest memory.
PowerPC kernels require this data structure, and even when firmware runs
in the guest, you still need to tell the firmware what the system layout
is, and the device tree is an obvious candidate...

For x86, maybe it doesn't make sense to have in-guest BIOS split a
qemu-provided device tree into all the nasty BIOS data structures, but I
just wanted to give you an idea of how this could be used on multiple
architectures.

-- 
Hollis Blanchard
IBM Linux Technology Center