[libvirt] [RFC] kvm: x86: export vCPU halted state to sysfs

[Viktor Mihajlovski]

On 02.02.2018 16:22, Luiz Capitulino wrote:
> On Fri, 2 Feb 2018 16:08:25 +0100
> Viktor Mihajlovski <mihajlov at linux.vnet.ibm.com> wrote:
> 
>>>> A disabled guest CPU is represented as halted in the QEMU object model
>>>> and can therefore be identified by the QMP query-cpus command.
>>>>
>>>> The initial patch proposal to expose this via virsh vcpuinfo was not
>>>> considered to be desirable because there was a concern that legacy
>>>> management software might be confused seeing halted vcpus. Therefore the
>>>> state information was added to the cpu domain statistics.
>>>>
>>>> One issue we're facing is that the semantics of "halted" are different
>>>> between s390 and at least x86. The question might be whether they are
>>>> different enough to grant a specific "disabled" indicator.  
>>>
>>> From your description, it looks like they are completely
>>> different.  On x86, a CPU that is online and in use can be moved
>>> between halted and non-halted state many times a second.
>>>
>>> If that's the case, we can probably fix this without breaking
>>> existing code: explicitly documenting the semantics of
>>> "vcpu.<n>.halted" at virConnectGetAllDomainStats() to mean "not
>>> online" (i.e. the s390 semantics, not the x86 one), and making
>>> qemuMonitorGetCpuHalted() s390-specific.
>>>
>>> Possibly a better long-term solution is to deprecate
>>> "vcpu.<n>.halted" and make "vcpu.<n>.state" work correctly on  
>>> s390>  
>> As it seems that nobody was ever *really* interested in x86.halted, one
>> could also return 0 unconditionally there (and for other
>> expensive-to-query arches)?
> 
> The most important question I have is: does this solution satisfy the
> needs of upper management? That is, if we implement the solution suggested
> by Eduardo than the feature of automatically hotplugging more CPUs
> will only work for s390. Is this OK?
> 
> If yes, then I think this is the best solution. And the next question
> would be: Viktor, can you change this in libvirt while we fix query-cpus
> in QEMU?
> 
The latest proposal was to use a flag for query-cpus (like full-state)
which would control the set of properties queried and reported. If this
is the way we decide to go, I can make the necessary changes in libvirt.
> Btw, I guess OpenStack ran into this issue just because this field
> slipped into domstats API and ceilometer issues that command...
> 
>>> It would be also interesting to update QEMU QMP documentation to
>>> clarify the arch-specific semantics of "halted".
>>>   
>>
>>
> 


-- 
Regards,
 Viktor Mihajlovski