Re: [Qemu-devel] [PATCH v2 1/3] pc-dimm: add a function to calculate VM's current RAM size

[Eric Blake <eblake@redhat.com>]

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On 11/19/2014 09:06 AM, Michael S. Tsirkin wrote:

>>>> This affects QMP right?
>>>
>>> I think later patches will tell how. CC'ing Eric.
>>
>> As far as I can tell, this is just correcting a reporting issue; the
>> existing QMP commands/events for tracking balloon size will now properly
>> account for hotplugged memory.
>>
>> What I don't know is if this change in semantics will affect any users.
>>  Libvirt is not yet supporting memory hotplug, so ideally, fixing this
>> bug before libvirt uses memory hotplug means libvirt will never have to
>> worry about qemu versions that do incorrect reporting.
>>
>> The alternative is to declare that the existing QMP commands cannot
>> change in semantics for the existing members that it reports, and must
>> instead report additional dictionary members describing the amount of
>> hot-plugged memory, and then require that the client add the numbers
>> together itself.  That sounds mean to the client, so I'm hoping we don't
>> have to go there.
> 
> 
> IOW you ack this patch for 2.2?
> 

Is memory hotplug one of the new features in 2.2?  If so, then yes, we
should get its semantics right from the start (this is a bug fix to
avoid a release with broken semantics).  On the other hand, if hotplug
existed in 2.1, then we already have a release with odd semantics, so
delaying this fix until 2.3 and leaving 2.2 with the same odd semantics
would not hurt, and it then becomes a judgment call of whether we are
rushing in a possibly incomplete solution by trying to get this into
2.2. (Sorry I haven't been following the history of memory hotplug closer)


-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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