Re: RFC qdev path semantics

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2010-06-17 at 23:01 +0100, Paul Brook wrote:
> > > ### Paul proposes to require all buses to define bus addresses.  Make
> > >     one up if necessary.
> > 
> > That seems arbitrary and prone to breakage.  How do we handle a subtle
> > change in device instantiation order and still allow migration?  If by
> > code change or command line ordering my frobnitz moves from:
> > 
> > /i440FX-pcihost/pci.0/PIIX3/@01.0/isa.0/0
> > 
> > to
> > 
> > /i440FX-pcihost/pci.0/PIIX3/@01.0/isa.0/1
> 
> Two things are apparent here. 
> (a) You've clearly misunderstood the proposals. The paths above make no sense.

Sorry, hastily created paths.  Though yes, I am a little unclear of the
proposal, feel free to code up how it should work.  I hope the other
follow-up I just sent is more correct.

> (b) You've picked a particularly poor definition of device address for the ISA 
> bus. We can do much better than device creation order.

Ok, how?

> > ...
> > I can live with PATH/@BUS-ADDR if it's still felt that
> > PATH/IDENT@BUS-ADDR isn't canonical.  What that means is that I'll
> > probably code up vmstate and ramblocks to append IDENT themselves to
> > keep all the goodness of having per PATH/IDENT namespaces.
> 
> As discussed elsewhere in this thread, addition of IDENT to the device path is 
> neither necessary nor sufficient for migration.
> 
> I really feel like we're going round in circles here.

Um, I believe I just agreed to remove IDENT from the canonical path and
append it in a usage specific way.  I think I've cited a couple relevant
examples of how this can improve the robustness of migration and I have
yet to hear anything but conjecture that this is only a false sense of
security.

Alex