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

[Peter Xu <peterx@redhat.com>]

On Mon, Feb 15, 2016 at 10:52:01AM +0100, Markus Armbruster wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > For ARM platform, we still do not have any interface to query
> > whether current QEMU/host support specific GIC version. This
> > patchset is trying to add one QMP interface for that. By querying
> > the GIC capability using the new interface, one should know exactly
> > what GIC version(s) the platform will support. The capability bits
> > will be decided by both QEMU and host kernel.
> >
> > The current patchset only provides interface for review. Its handler
> > is a fake one which returns empty always.
> >
> > The command interface I am planning to add is something like this:
> >
> > -> { "execute": "query-gic-capability" }
> > <- { "return": [ "gicv2", "gicv2-kvm", "gicv3-kvm" ] }
> >
> > Currently, all the possible supported GIC versions are:
> >
> > - gicv2:      GIC version 2 without kernel IRQ chip
> > - gicv2-kvm:  GIC version 2 with kernel IRQ chip
> > - gicv3:      GIC version 3 without kernel IRQ chip (not supported)
> > - gicv3-kvm:  GIC version 3 with kernel IRQ chip
> >
> > Since "gicv3" is still not supported (to use GICv3, kernel irqchip
> > support is required for now, which corresponds to "gicv3-kvm"),
> > currently the maximum superset of the result should be:
> >
> > ["gicv2", "gicv2-kvm", "gicv3-kvm"]
> >
> > Please help review whether the interface suits our need, also please
> > point out any error I have made.
> 
> Adding ad hoc queries as we go won't scale.  Is there really no generic
> way to get this information, e.g. with qom-get?

Haven't used "qom-get" before, but it seems to fetch one property
for a specific object. If so, will it be strange to hide some
capability bits into every GIC objects (though there is possibly
one object)?

I agree that we should keep the interface as simple as
possible. I see that there are already commands that works just like
this one, which is to query some capabilities from QEMU, like:

- query-dump-guest-memory-capability
- query-migrate-capabilities

So... besides the original proposal, what about adding a generic QMP
command to query all kinds of capabilities (and let GIC be the first
item)? Or any other way to avoid adding a new command?

Thanks.
Peter

> 
> > One question: how should I make this command "ARM only"? I see that
> > in qmp-commands.hx, I can use something like "#if defined
> > TARGET_ARM" to block out ARM specified commands, however how should
> > I do the similiar thing in qapi-schema.json?
> 
> Have a look at the #if in qmp-commands.hx.