Re: [Qemu-devel] [RFC/PATCH 0/1] cpu hotplug for s390

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 03 May 2013 09:50:50 -0400
"Jason J. Herne" <jjherne@linux.vnet.ibm.com> wrote:

> I've done some investigating into using the device_add hmp/qmp command 
> to support hot-plugging cpus on S390.  The alternative suggestion was to 
> simply use a new cpu_add hmp/qmp command.
> 
> device_add accepts all of the same options as the -device command line 
> parameter takes.  This would imply that to hot-plug cpu's using device 
> add we would need to allow command line arguments of type "-device cpu". 
>   All of the implications of this are not currently clear to me.  How 
> would this interact with the -smp option, for example, how many cpus are 
> created in this case:
>      qemu -smp 2 -device cpu,id=cpu0 -device cpu,id=cpu1, -device
> cpu,id=cpu2
> Is -smp invalid when cpu devices are specified?  We would have to fill 
> the smp_cpus variable after all (cpu) devices have been parsed.
I haven't looked using -device in case of x86 it would require some extra
work besides planned properties+classes to make it usable.
However device_add for your case should be usable and there is no need to use
-device with it. 
-smp x,maxcpus=y will tell board how many CPUs 'x' should be created and
'y' - 'x' how many extra CPUs could be added later.

> 
> Since device_add requires a QOM object name (driver parameter) we
> seem to have
> two choices.
>      1. device_add cpu
>      2. device_add s390-cpu
> But "cpu" is actually an abstract QOM class and cannot be instantiated 
> by object_new("cpu") as is done in device_add processing.  So we need to 
> use "s390-cpu".  This adds an architecture specific flavor to cpu
> hotplug. I would think we'd want to avoid that somehow.  perhaps we 
> simply "translate" that parameter during early device_add processing?
that is exactly intention to use architecture specific types there so that
one could specify what kind/flavor of CPU needs to be added using type names.

> Another issue is that the s390-cpu QOM object class is a child of 
> "main-system-bus".  This bus does not support hotplug: 
> sysbus->allow_hotplug=0. In order to implement cpu hotplug we would need 
> to either switch sysbus->allow_hotplug to 1, or the s390-cpu QOM object 
> class would need to move to a bus that supports hotplug.  I'm not sure 
> what the implications of either choice would be.
For x86 CPU was moved to ICC bus that existed in real hardware, probably there
is similar bus for s390. Alternatively you can consider if s390-cpu could be
bus-less, since commit 2f7bd829db it's possible device_add bus-less devices.

> I'm interested in thoughts and comments.  Thanks!
> 
> -- 
> -- Jason J. Herne (jjherne@linux.vnet.ibm.com)
> 


-- 
Regards,
  Igor