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

[Bharata B Rao]

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 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.