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 11:11:31AM +0100, Markus Armbruster wrote:
> David Gibson <david@gibson.dropbear.id.au> writes:
> 
> > 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.
> 
> I'm not against changing it, I just want it changed intentionally rather
> than by accidental side effect :)
> 
> If you tell me you want null here going forward, I'll make sure it gets
> changed to null in the next development cycle, with a nice commit
> message.

I would like it to be null (or simply missing) in future.

> If you want it to be null in 2.5, NAK "[PATCH for-2.5 2/3] spapr_drc:
> Change value of property "fdt" from null back to {}".  It'll remain an
> implicit change then, not documented in commit messages.  I expect we'll
> eventually get a patch similar to the NAKed one regardless, because
> we'll tighten up the visitor contracts, and returning without visiting
> anything should become a programming error then.

But leaving it as is for 2.5 is fine.


-- 
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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