Re: [Qemu-devel] Posible regressions around spapr-dr-connector property drc-connector[]

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Dec 04, 2015 at 09:05:51AM +0100, Markus Armbruster wrote:
> David Gibson <david@gibson.dropbear.id.au> writes:
> 
> > On Thu, Dec 03, 2015 at 04:30:31PM +0100, Markus Armbruster wrote:
> >> 1. Before commit 94649d4 "spapr: Don't use QOM [*] syntax for DR
> >>    connectors", the indexes were small integers:
> >> 
> >>        (qemu) info qom-tree
> >>        /machine (pseries-2.4-machine)
> >>          /unattached (container)
> >>            [...]
> >>            /device[5] (spapr-pci-host-bridge)
> >>              /pci@800000020000000.mmio[0] (qemu:memory-region)
> >>              /pci@800000020000000.mmio-alias[0] (qemu:memory-region)
> >>              /pci@800000020000000.io[0] (qemu:memory-region)
> >>              /pci@800000020000000.io-alias[0] (qemu:memory-region)
> >>              /pci.0 (PCI)
> >>              /pci@800000020000000.iommu-root[0] (qemu:memory-region)
> >>              /dr-connector[0] (spapr-dr-connector)
> >>              /dr-connector[1] (spapr-dr-connector)
> >>              /dr-connector[2] (spapr-dr-connector)
> >>              [...]
> >> 
> >>    Since then, they're big ones:
> >> 
> >>              /dr-connector[1073741824] (spapr-dr-connector)
> >>              /dr-connector[1073741825] (spapr-dr-connector)
> >>              /dr-connector[1073741826] (spapr-dr-connector)
> >> 
> >>    The commit message doesn't quite spell out this change, and I'm
> >>    therefore double-checkint it's intentional.  Is it?
> >
> > Yes, it's intentional.  The small integers were arbitrarily allocated
> > by the QOM magic [*] code, whereas the big integers are actually
> > meaningful values (essentially the DRC's global ID for the dynamic
> > reconfiguration hypervisor interfaces).
> 
> Good.
> 
> >> 2. Before commit 6c2f9a1 "qapi: Make output visitor return qnull()
> >>    instead of NULL", qom-get returned {}:
> >> 
> >>    Since then, it returns null:
> >> 
> >>        QMP> { "execute": "qom-get", "arguments": { "path": "/machine/unattached/device[5]/dr-connector[1073741950]", "property": "fdt" } }
> >>        {"return": null}
> >> 
> >>    Does anyone care?
> >
> > Hm, I'm guessing this is a case where fdt is NULL internally.  Which I
> 
> Yes.
> 
> > think will happen before a device gets hotplugged into the DRC.  In
> > that case null seems more correct to me than {}, since {} would also
> > be what's shown for a present-but-empty device tree.
> 
> It was {} in 2.4.  Changing it to null so we can distingish "nothing"
> from "empty" is an incompatible change.  May make sense anyway, but I
> can't judge it.

Strictly speaking it's an incompatible change, yes.  But I find it
hard to imagine anything would be relying on the {} behaviour.  This
property is essentially a debugging interface to start with, and the
missing / empty case is examining it in a state that's unlikely to be
interesting.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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