On Mon, Feb 29, 2016 at 04:42:58PM +0100, Igor Mammedov wrote: > On Thu, 25 Feb 2016 14:52:06 -0300 > Eduardo Habkost <ehabk...@redhat.com> wrote: > > > On Wed, Feb 24, 2016 at 03:42:18PM +0100, Igor Mammedov wrote: > > > On Tue, 23 Feb 2016 18:26:20 -0300 > > > Eduardo Habkost <ehabk...@redhat.com> wrote: > > > > > > > On Tue, Feb 23, 2016 at 10:46:45AM +0100, Igor Mammedov wrote: > > > > > On Mon, 22 Feb 2016 13:54:32 +1100 > > > > > David Gibson <da...@gibson.dropbear.id.au> wrote: > > > > [...] > > > > > > This is why Eduardo suggested - and I agreed - that it's probably > > > > > > better to implement the "1st layer" as an internal > > > > > > structure/interface > > > > > > only, and implement the 2nd layer on top of that. When/if we need > > > > > > to > > > > > > we can revisit a user-accessible interface to the 1st layer. > > > > > We are going around QOM based CPU introspecting interface for > > > > > years now and that's exactly what 2nd layer is, just another > > > > > implementation. I've just lost hope in this approach. > > > > > > > > > > What I'm suggesting in this RFC is to forget controversial > > > > > QOM approach for now and use -device/device_add + QMP introspection, > > > > > > > > > > > > > You have a point about it looking controversial, but I would like > > > > to understand why exactly it is controversial. Discussions seem > > > > to get stuck every single time we try to do something useful with > > > > the QOM tree, and I don't undertsand why. > > > Maybe because we are trying to create a universal solution to fit > > > ALL platforms? And every time some one posts patches to show > > > implementation, it would break something in existing machine > > > or is not complete in terms of how interface would work wrt > > > mgmt/CLI/migration. > > > > That's true. > > > > > > > > > > > > > > i.e. completely split interface from how boards internally implement > > > > > CPU hotplug. > > > > > > > > A QOM-based interface may still split the interface from how > > > > boards internally implement CPU hotplug. They don't need to > > > > affect the device tree of the machine, we just need to create QOM > > > > objects or links at predictable paths, that implement certain > > > > interfaces. > > > Beside of not being able to reach consensus for a long time, > > > I'm fine with isolated QOM interface if it allow us to move forward. > > > However static QMP/QAPI interface seems to be better describing and > > > has better documentation vs current very flexible poorly self-describing > > > QOM. > > > > You have a good point: QMP is more stable and better documented. > > QOM is easier for making experiments, and I would really like to > > see it being used more. But if we still don't understand the > > requirements enough to design a QMP interface, we won't be able > > to implement the same functionality using QOM either. > > > > If we figure out the requirements, I believe we should be able to > > design equivalent QMP and QOM interfaces. > So not to stall CPU hotplug progress, I'd start with stable QMP query > interface for general use, leaving experimental QOM interface for later > as difficult to discover and poorly documented one from mgmt pov, > meaning mgmt would have to: > - instantiate a particular machine type to find if QOM interface is > supported, > i.e. '-machine none' won't work with it as it's board depended VS static > compile time qapi-schema in QMP case > - execute a bunch of qom-list/qom-read requests over wire to enumerate/query > objects starting at some fixed entry point (/machine/cpus) VS a single > command that does 'atomic' enumeration in QMP case.
That sounds reasonable to me.
However, before even that, I think we need to work out exactly what
device_add of a multi-thread cpu module looks like. I think that's
less of a solved problem than everyone seems to be assuming.
--
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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