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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 2 Feb 2018 12:50:14 -0200
Eduardo Habkost <ehabkost@redhat.com> 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.

Agree. The only thing I'm unsure about is that, is the performance
issue only present in x86? If yes, then do we deprecate it only
for x86 or for all archs? Maybe for all archs, otherwise this has
the potential to turn into a mess.

> 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 guess we can't remove fields, but maybe we could always return
"running" and skip interrupting the vCPU threads.