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

From: Peter Krempa
Subject: Re: [Qemu-devel] [PATCH 1/3] qapi: Report support for -device cpu hotplug in query-machines
Date: Fri, 24 Jun 2016 07:41:11 +0200
User-agent: Mutt/1.5.24 (2015-08-30)

On Fri, Jun 24, 2016 at 14:56:51 +1000, David Gibson wrote:


> > 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?

It is not strictly necessary for us to have this in the context of
usability. If the user requests using the new hotplug feature we will
try it unconditionally and call query-hotpluggable-cpus before even
starting guest execution. A failure to query the state will then result
in termination of the VM.

It is necessary though to report the availability of the feature to the
user via our domain capabilities API which some higher layer management
apps use to make decisions.

This would also be necessary if we wanted to switch by default to the
new approach, but that's not really possible as libvirt tries to
guarantee that a config valid on certain version will be still valid
even when it was migrated to a newer version and then back.

My current plan is to start qemu with -smp cpus=1,... and then call
query-hotpluggable-cpus and then hotplug all of them until the requested
configuration is satisfied. This approach is necessary so that we can
query for the model and topology info so that we don't need to
re-implement all the numbering and naming logic from qemu.

Additionally this will require us to mark one CPU as non-hotpluggable as
-smp cpus=0,maxcpus=10 is basically translated to -smp


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