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Re: [Qemu-devel] [PATCH 1/3] qapi: Report support for -device cpu hotplu

From: David Gibson
Subject: Re: [Qemu-devel] [PATCH 1/3] qapi: Report support for -device cpu hotplug in query-machines
Date: Fri, 24 Jun 2016 14:56:51 +1000

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?

David Gibson <address@hidden>
Senior Software Engineer, Virtualization, Red Hat

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