Re: [PATCH v2] ARM: DT: Add binding for GIC virtualization extentions (VGIC)

[Marc Zyngier <marc.zyngier-5wv7dgnIgG8@public.gmane.org>]

On Wed, 9 May 2012 20:27:49 +0000, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> On Wednesday 09 May 2012, Marc Zyngier wrote:
>> On Wed, 9 May 2012 19:21:42 +0000, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
>> > On Wednesday 09 May 2012, Marc Zyngier wrote:
>> >> 
>> >> The GICv2 can have virtualization extension support, consisting
>> >> of an additional set of registers and interrupts. Add the necessary
>> >> binding to the GIC DT documentation.
>> >> 
>> >> Signed-off-by: Marc Zyngier <marc.zyngier-5wv7dgnIgG8@public.gmane.org>
>> > 
>> > Would it make sense to add a way to detect whether a GIC is virtual
>> > or real? Maybe an optional empty "virtual-gic" property or an
>> > additional
>> > "compatible" value. Even if we don't need it now, it might come in
>> > handy
>> > if we require it already.
>> 
>> I don't really see a need for this. When running on a virtual machine,
>> the
>> kernel cannot tell if this is the real thing or not (the virtual CPU
>> interface looks exactly like the normal one once mapped into the guest
>> address space).
>> 
>> Or maybe I just didn't get your use case?
> 
> Well, one difference seems to be the VGIC maintainance interrupt that
may
> or may not be present, and another one the additional registers. Of
course
> you can imply the type of GIC from the presence of this extra data,
> but I think it would be better to make it explicit.

Ah, I see what you mean. But these registers and interrupt are actually
only visible on the hypervizor side. The guest shouldn't see any of this.

I suppose we could have a "arm,has-virt-extensions" property in devices
that implement the virtualization extensions (GIC, timers...).

What do you think?

        M.
-- 
Fast, cheap, reliable. Pick two.