Re: [Qemu-devel] [PATCH] S390: Expose s390-specific CPU info

[Luiz Capitulino <lcapitulino@redhat.com>]

On Thu, 8 Feb 2018 16:21:26 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> On Thu, 8 Feb 2018 09:09:04 -0500
> Luiz Capitulino <lcapitulino@redhat.com> wrote:
> 
> > On Thu,  8 Feb 2018 10:48:08 +0100
> > Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com> wrote:
> >   
> > > Presently s390x is the only architecture not exposing specific
> > > CPU information via QMP query-cpus. Upstream discussion has shown
> > > that it could make sense to report the architecture specific CPU
> > > state, e.g. to detect that a CPU has been stopped.    
> > 
> > I'd very strongly advise against extending query-cpus. Note that the
> > latency problems with query-cpus exists in all archs, it's just a
> > matter of time for it to pop up for s390 use-cases too.
> > 
> > I think there's three options for this change:
> > 
> >  1. If this doesn't require interrupting vCPU threads, then you
> >     could rebase this on top of query-cpus-fast  
> 
> From my perspective, rebasing on top of query-cpus-fast looks like a
> good idea. This would imply that we need architecture-specific fields
> for the new interface as well, though.

That's not a problem. I mean, to be honest I think I'd slightly prefer
to keep things separate and add a new command for each arch that needs
its specific information, but that's just personal preference. The only
strong requirement for query-cpus-fast is that it doesn't interrupt
vCPU threads.

> 
> > 
> >  2. If you plan to keep adding s390 state/registers to QMP commands,
> >     then you could consider adding a query-s390-cpu-state or add
> >     a query-cpu-state command that accepts the arch name as a parameter  
> 
> Personally, I don't see a need for more fields. But maybe I'm just
> unimaginative.
> 
> > 
> >  3. If you end up needing to expose state that actually needs an
> >     ioctl, then we should consider porting info registers to QMP
> >   
> > > 
> > > With this change the output of query-cpus will look like this on
> > > s390:
> > > 
> > >     [{"arch": "s390", "current": true,
> > >       "props": {"core-id": 0}, "cpu_state": "operating", "CPU": 0,
> > >       "qom_path": "/machine/unattached/device[0]",
> > >       "halted": false, "thread_id": 63115},
> > >      {"arch": "s390", "current": false,
> > >       "props": {"core-id": 1}, "cpu_state": "stopped", "CPU": 1,
> > >       "qom_path": "/machine/unattached/device[1]",
> > >       "halted": true, "thread_id": 63116}]
> > > 
> > > Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>  
>