[libvirt] [RFC] Per machine type qemu capability detection

[Peter Krempa]

On 04/04/13 18:22, Daniel P. Berrange wrote:
> On Thu, Apr 04, 2013 at 06:20:05PM +0200, Peter Krempa wrote:
>> Hi,
>>
>> qemu recently added a qmp/hmp command ("get-cpu-max") to determine
>> maximum number of supported CPUs for a VM. The output of this
>> command depends on the machine type that is used when it is called.
>> The problem here is that libvirt uses "-M none" while detecting
>> capabilities and with that machine type the get-cpu-max command
>> returns always 1.
>>
>> My question is: Is it worthwhile to do per machine type capability
>> detection? It definitely isn't worth doing just for the get-cpu-max
>> support, but it might be needed for Q35 or other stuff that may
>> depend on the actual machine type used.
>>
>> Pros of this approach:
>> ----------------------
>> - (possibly) more accurate capabilities
>>
>>
>> Cons:
>> -----
>> - ~10x more qemus started on libvirtd startup (one per machine type)
>
> That "con" simply rules it out entirely, not matter what "pro"
> points there may be I'm afraid.

Yes, that's why I'm asking if there isn't a better use case that could 
justify the overhead.
>
> For your example here though, why do we need a capabiltiy for that?
> Can't we just call get-cpu-max from the specific libvirt API calls
> that actually want that info.

I planed to use it to validate configuration when parsing domain XMLs 
where it would come handy to have the data pre-cached. Also I don't 
think it's wise to invoke a qemu when a domain XML is being parsed to 
find out if the configuration is valid.

Other than validating XML config, this information can only be 
meaningfully used with CPU hotplug. Other use cases aren't really useful 
as qemu either checks the number directly at start of the guest or 
simply isn't running at the time the information is required.

>
> Daniel
>

Peter