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

[Eduardo Habkost]

On Fri, Feb 02, 2018 at 02:53:50PM +0100, Viktor Mihajlovski wrote:
> On 01.02.2018 21:26, Eduardo Habkost wrote:
> > On Thu, Feb 01, 2018 at 09:15:15PM +0100, Radim Krčmář wrote:
> >> 2018-02-01 12:54-0500, Luiz Capitulino:
> >>>
> >>> Libvirt needs to know when a vCPU is halted. To get this information,
> >>
> >> I don't see why upper level management should care about that, a single
> >> bit about halted state that can be incorrect at the time it is processed
> >> seems of very limited use.
> > 
> > I don't see why, either.
> > 
> > I'm CCing libvir-list and the people involved in the code that
> > added halt state to libvirt domain statistics.
> > 
> I'll try to explain the motivation for the "halted" state exposure and
> why it ended int the libvirt domain stats.
> 
> s390 CPUs can be present in a system (e.g. after being hotplugged) but
> be offline (disabled) in which case they are not used by the operating
> system. In Linux disabled CPUs show a value of '0' in
> /sys/devices/system/cpu/cpu<n>/online.
> 
> Higher level management software (on top of libvirt) can take advantage
> of knowing whether a guest CPU is online and thus used or not.
> Specifically it might not make sense to plug more CPUs if the guest OS
> isn't using the CPUs at all.

Wasn't this already represented on "vcpu.<n>.state"?  Why is
"vcpu.<n>.halted" needed?

> 
> 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.

It would be also interesting to update QEMU QMP documentation to
clarify the arch-specific semantics of "halted".

-- 
Eduardo