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

[Luiz Capitulino]

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?

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".
> >   
> 
>