Re: [Qemu-devel] [PATCH] spapr: Don't support query-hotpluggable-cpus on earlier pseries machine types

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 2 Aug 2016 16:20:50 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Aug 02, 2016 at 10:34:34AM +0530, Bharata B Rao wrote:
> > On Tue, Aug 02, 2016 at 02:25:08PM +1000, David Gibson wrote:  
> > > On Power, support for vCPU hotplug is new in qemu 2.7.  However, we
> > > currently implement the query_hotpluggable_cpus hook the same for all
> > > pseries machine type versions.
> > > 
> > > However, the old-style CPU initialization doesn't work with the new query
> > > code, meaning that attempting to use query-hotpluggable-cpus on a
> > > pseries-2.6 or earlier VM will cause qemu to SEGV.
> > > 
> > > This fixes the problem by simply disabling the hook for earlier machine
> > > types.  
> > 
> > I had sent a patch to fix this and a couple of other related issues
> > some time back and it indeed was accepted into your ppc-for-2.7 branch.
> > 
> > https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg01539.html
> > 
> > Only now I am realizing that somehow that patch didn't make it to mainline.  
> 
> Oh.. good point.  Sorry, that one somehow slipped through the cracks.
> 
> So, the remaining question is, what's the preferred behaviour for
> older machine types:
> 
>   1) should query-hotpluggable-cpus give an error, the same as it does
>      on machine types which have never supported it (this is what my
>      patch does)
> 
> or
> 
>   2) Should query-hotpluggable-cpus succeed, but return an empty list?
>      (this is what Bharata's patch does)
> 
> Igor and / or Peter, do you have an opinion on which behaviour is preferable?
> 

If it doesn't have any output then it makes sense to set handler to NULL
so it would return error.