Re: [Qemu-devel] [PATCH 1/3] qapi: Report support for -device cpu hotplug in query-machines

[Igor Mammedov]

On Fri, 24 Jun 2016 14:56:51 +1000
David Gibson <address@hidden> wrote:

> On Thu, 23 Jun 2016 21:49:25 -0600
> Eric Blake <address@hidden> wrote:
> 
> > On 06/23/2016 08:56 PM, David Gibson wrote:
> > > On Thu, 23 Jun 2016 22:23:23 +0200
> > > Peter Krempa <address@hidden> wrote:
> > >   
> > >> For management apps it's very useful to know whether the selected
> > >> machine type supports cpu hotplug via the new -device approach.
> > >> Using the presence of 'query-hotpluggable-cpus' is enough for a
> > >> withess. 
> > 
> > > 
> > > I'd been under the impression that there was a general way of
> > > detecting the availability of a particular qmp command.  Was I
> > > mistaken?  
> > 
> > You are correct - query-commands says whether
> > 'query-hotpluggable-cpus' exists as a command.  But that is
> > insufficient.  See my review, or the v2 patch, where the above poor
> > wording was corrected to say what was really meant: knowing whether
> > query-hotpluggable-cpus exists is insufficient to tell you whether
> > a given cpu type can be hotplugged.  So adding one more piece of
> > witness (for every type of cpu supported, we also advertise if it
> > is hotpluggable) is enough for libvirt to efficiently take
> > advantage of the new query-hotpluggable-cpus
> > command.
> 
> Ah, right.  Or to put it another way, the availability of
> query-hotpluggable-cpus is global across qemu, whereas actually being
> able to use it for hotplug is per machine type.
> 
> Would it be possible to do this instead by attempting to invoke
> query-hopluggable-cpus and seeing if it returns any information?
This sounds like a better way, for x86 we can set
query-hotpluggable-cpus hook to NULL for old machine types so that
it would return error that it's not supported.

> 
>