Re: [Qemu-devel] [RFC PATCH 0/2] ARM: add QMP command to query GIC version

[Peter Xu <peterx@redhat.com>]

On Thu, Feb 18, 2016 at 06:10:21PM +0100, Andrea Bolognani wrote:
> On Thu, 2016-02-18 at 17:52 +0100, Andrew Jones wrote:
> > > Is this work on any of our todo list (or anyone has started the
> > > prototyping)?
> > > 
> > > It seems reasonable to provide such a generic interface, rather than
> > > adding a "query-gic-capability" for GIC versions only. The problem
> > > is that, I am not sure how eagerly we are wanting this GIC
> > > interface, and when will this framework be there in QOM.
> > 
> > We want it eagerly :-) This type of a rabbit hole is likely why Daniel
> > was suggesting we do more in libvirt. I'm still not sure we want to
> > probe both kvm and qemu from libvirt though, so I'm still in favor of
> > an improved qemu probing method being worked out.
> > 
> > I don't know what the policy is for deprecating QMP commands, but I
> > wonder if we can't introduce a QMP command now, and then, after working
> > out the QOM extensions, we could shift to it, deprecating this QMP
> > command and any others that would no longer be needed.
> 
> AFAIK, the current situation of libvirt passing the GIC version to
> QEMU and simply reporting in case of failure is not unprecedented
> and there are a few cases where probing in advance would simply not
> be feasible.
> 
> Any probing code added to libvirt would have to be kept around
> forever to ensure compatibility with current QEMU versions, so it
> should IMHO be seen as a last resort in case we can't live without
> GIC version probing while it's being implemented, properly, in QEMU.

If libvirt is the most possible consumer for the new command, I
think it might not be too hard to keep the compatibility of all
possible versions of QEMU. E.g., after we have got a better way to
query GIC version other than query-gic-capability, we can do
something like this in libvirt:

- try query-gic-capability
  - if supported -> [got GIC version]
  - if not supported -> try the new method
    - if supported -> [got GIC version]
    - if not supported -> [not support]

During the time when QEMU has both methods working (before
obsoleting the query-gic-capability QMP command), QEMU will make
sure querying in both way will get exactly the same results.

Does this work?

Thanks.
Peter