Re: [Qemu-devel] [PATCH v2] qmp: add query-cpus-fast

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 9 Feb 2018 15:50:00 +0100
Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com> wrote:

> On 09.02.2018 15:27, Eduardo Habkost wrote:
> [...]
> >> I'm keeping it mainly for s390. Viktor, libvirt is still using
> >> this field in s390, no?
> >>
> >> Dropping halted and having management software still using query-cpus
> >> because of halted would be a total failure of query-cpus-fast.  
> > 
> > If I understood correctly, the CpuInfoS390::cpu_state field added
> > by Viktor in another patch[1] would replace "halted" for the s390
> > case.  
> Right, CPUState.halted is derived from CPUS390XState.cpu_state on s390:
> A cpu_state of CPU_STATE_STOPPED or CPU_STATE_CHECK_STOPPED results in
> halted = true. This derivation can be done by libvirt for s390.
> > 
> > I'm assuming QEMU will be able to return that field without
> > interrupting the VCPUs.  Viktor, is that correct?
> > 
> > [1] https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg02032.html
> >   
> That's correct.
> >>  
> >>>> Also, the code that sets/clears cpu->halted is target-specific,
> >>>> so I wouldn't be so sure that simply checking for
> >>>> !kvm_irqchip_in_kernel() is enough on all targets.  
> >>
> >> I checked the code and had the impression it was enough, but
> >> I don't have experience with other archs. So, would be nice
> >> if other archs maintainers could review this. I'll try to ping them.  
> > 
> > I think we need to take a step back and rethink:
> > 
> > 1) What the field is supposed to mean?  The semantics of "halted"
> >    are completely unclear.  What exactly we want to communicate
> >    to libvirt/management?
> > 2) On which cases the information (whatever it means) is really
> >    useful/important?  If you are excluding cases with in-kernel
> >    irqchip, you are already excluding most users.
> > 
> >   
> Given that nobody (including myself) sees a need for halted we can
> remove it for the fast version of query-cpus without surprising anyone.
> [...]

My conclusion too, I'll drop it.