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

From: Viktor Mihajlovski
Subject: Re: [Qemu-devel] [PATCH v2] qmp: add query-cpus-fast
Date: Fri, 9 Feb 2018 15:50:00 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

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:
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.

 Viktor Mihajlovski

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