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

On Fri, Feb 02, 2018 at 12:15:54PM -0200, Eduardo Habkost wrote: > 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". Any also especially clarify the awful performance implications of running this particular query command. In general I would not expect query-xxx monitor commands to interrupt all vcpus, so we should clearly warn about this !