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

[Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>]

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.
[...]

-- 
Regards,
 Viktor Mihajlovski