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

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Feb 02, 2018 at 03:07:18PM +0000, Daniel P. Berrangé wrote:
> 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.

This would work, too.  I would name it "full-state",
"extended-state" or something similar, though.  Not all
arch-specific data is expensive to fetch, and not all
non-arch-specific data is unexpensive.

But I'd like to confirm if it's OK to make existing non-optional
struct fields optional in the QAPI schema.  Markus, Eric?

-- 
Eduardo