Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus

[David Gibson]

On Mon, Feb 29, 2016 at 04:42:58PM +0100, Igor Mammedov wrote:
> On Thu, 25 Feb 2016 14:52:06 -0300
> Eduardo Habkost <address@hidden> 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 <address@hidden> 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 <address@hidden> 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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