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From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= <berrange@redhat.com>
Subject: Re: [RFC] kvm: x86: export vCPU halted state to sysfs
Date: Fri, 2 Feb 2018 15:07:18 +0000
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Cc: Luiz Capitulino <lcapitulino@redhat.com>,
        Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
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        Christian Borntraeger <borntraeger@de.ibm.com>,
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        Eric Blake <eblake@redhat.com>
To: Eduardo Habkost <ehabkost@redhat.com>
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On Fri, Feb 02, 2018 at 12:50:14PM -0200, 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?

I would expect that to not be OK.  A fully backwards compatible way to
deal with this would just be to add a flag to the query-cpus command
eg something like

    query-cpus arch-specific=false

to turn off all this arch specific state, and just report the cheap
generic info. If it defaults to arch-specific=true when omitted, then
there's no compat problems.

Regards,
Daniel
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