Re: [Qemu-devel] [RFC] kvm: x86: export vCPU halted state to sysfs

[Eric Blake <eblake@redhat.com>]

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On 02/02/2018 08:50 AM, Eduardo Habkost wrote:
> (CCing qemu-devel)
> 
> On Fri, Feb 02, 2018 at 09:21:59AM -0500, Luiz Capitulino wrote:
>> On Fri, 2 Feb 2018 14:19:38 +0000
>> Daniel P. Berrangé <berrange@redhat.com> wrote:
>>> On Fri, Feb 02, 2018 at 12:15:54PM -0200, Eduardo Habkost wrote:
> [...]
>>>> 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 !
>>
>> Or deprecate it...
> 
> We could deprecate the expensive fields on query-cpus, and move
> them to a more expensive query-cpu-state command.  I believe most
> users of query-cpus are only interested in qom_path, thread_id,
> and topology info.
> 
> Markus, Eric: from the QAPI point of view, is it OK to remove
> fields between QEMU versions, as long as we follow our
> deprecation policy?

Removing an output field outright may break a client that depended on
the field; so a deprecation period is definitely required there.  But it
is okay, documentation-wise, to state that a field is output always as 0
for back-compatibility reasons and that modern clients should ignore it
(which would then let old clients still parse the field, but no longer
see a non-zero value), whether or not we also pursue the deprecation
course and eventually remove the field after more releases.

See CpuInfo::current, for an example.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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