Re: [Qemu-devel] [RFC 0/2] cpu-add compatibility for query-hotpluggable-cpus implementations

[David Gibson]

On Tue, Jul 19, 2016 at 09:58:59AM +0530, Bharata B Rao wrote:
> On Mon, Jul 18, 2016 at 06:20:35PM +0200, Igor Mammedov wrote:
> > On Mon, 18 Jul 2016 17:06:18 +0200
> > Peter Krempa <address@hidden> wrote:
> > 
> > > On Mon, Jul 18, 2016 at 19:19:18 +1000, David Gibson wrote:
> > > > I'm not entirely sure if this is a good idea, and if it is whether
> > > > this is a good approach to it.  But I'd like to discuss it and see if
> > > > anyone has better ideas.
> > > > 
> > > > As you may know we've hit a bunch of complications with cpu_index
> > > > which will impose some limitations with what we can do with the new
> > > > query-hotpluggable-cpus interface, and we've run out of time to
> > > > address these in qemu-2.7.
> > > >
> > > > At the same time we're hitting complications with the fact that the
> > > > new qemu interface requires a new libvirt interface to use properly,
> > > > and that has follow on effects further up the stack.  
> > > 
> > > The libvirt interface is basically now depending on adding a working
> > > implementation for qemu or a different hypervisor. APIs without
> > > implementation are not accepted upstream.
> > > 
> > > It looks like there are the following problems which make the above
> > > hard:
> > > 
> > > First of the problem is the missing link between the NUMA topology
> > > (currently confirured via 'cpu id' which is not linked in any way to the
> > > query-hotpluggable-cpus entries). This basically means that I'll have to
> > > re-implement the qemu numbering scheme and hope that it doesn't change
> > > until a better approach is added.
> > with current 'in order' plug/unplug limitation behavior is the same as
> > for cpu-add (wrt x86) so device_add could be used as direct replacement
> > of cpu-add in NUMA case.
> > 
> > Numa node to CPU in query-hotpluggable-cpus a missing part
> > but once numa mapping for hotplugged CPUs (which is broken now) is fixed
> > (fix https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg00595.html)
> > I'll be ready to extend x86.query-hotpluggable-cpus with numa mapping
> > that -numa cpus=1,2,3... happened to configure.
> > (note: that device_add cpu,node=X that doesn't match whatever has been
> > configured with -numa cpus=... will rise error, as numa configuration
> > is static and fixed at VM creation time, meaning that "node" option
> > in query-hotpluggable-cpus is optional and only to inform users to
> > which node cpu belongs)
> > 
> > > Secondly from my understanding of the current state it's impossible to
> > > select an arbitrary cpu to hotplug but they need to happen 'in order' of
> > > the cpu id pointed out above (which is not accessible). The grand plan
> > > is to allow adding the cpus in any order. This makes the feature look
> > > like a proof of concept rather than something useful.
> 
> > having out-of-order plug/unplug would be nice but that wasn't
> > the grand plan. Main reason is to replace cpu-add with 'device_add cpu' and
> > on top of that provide support for 'device_del cpu' instead of adding 
> > cpu-del
> > command.
> > And as result of migration to device_add to avoid changing -smp to match
> > present cpus count on target and reuse the same interface as other devices.
> > 
> > We can still pick 'out of order' device_add cpu using migration_id patch
> > and revert in-order limit patch. It would work for x86,
> > but I think there were issues with SPAPR, that's why I'm in favor of
> > in-order limit approach.
> 
> Not that the migration_id patch doesn't work for sPAPR, but it was felt
> that having too many IDs (cpu_dt_id, arch_id, migration_id) is not
> good/idea/preferable and could cause confusion.

I was also concerned that adding another id would be yet another layer
of things we needed to maintain compatibility with in future.

> I am not clear as to why limiting the out-of-order hotplug is a show
> stopper for libvirt actually. Isn't that how it is for cpu-add currently ?
> 
> Regards,
> Bharata.
> 

-- 
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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