Re: [RFC PATCH-for-9.1? 1/2] target/arm: Move qmp_query_gic_capabilities() to hw/intc/

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Aug 08, 2024 at 12:32:35PM +0100, Peter Maydell wrote:
> On Thu, 8 Aug 2024 at 12:23, Daniel P. Berrangé <berrange@redhat.com> wrote:
> > On Thu, Aug 08, 2024 at 11:15:17AM +0100, Peter Maydell wrote:
> > > It's not possible to use KVM with that machine type, so the
> > > question is a bit moot. (This also indicates that the
> > > interface is not very helpful -- it purports to tell the
> > > management layer whether it can use an accelerated in-kernel
> > > GIC, but because it doesn't specifiy the board type there's
> > > no way to provide an accurate answer. It would be useful
> > > to know exactly what libvirt/etc actually use this for...)
> >
> > Libvirt uses this exclusively with the arm 'virt' machine type.
> >
> > If the user didn't express any GIC preference, then if KVM is in use,
> > we'll pick the highest GIC version QEMU reports as supported.
> 
> You can get that without querying QEMU by asking for 'gic-version=max'
> if you like.

This isn't in the VM startup path. It is when we expand the user
provided XML config into an ABI stable XML config by filling in
the blanks left by the user. So we need to actually query the
values available.

> > If TCG
> > is in use we'll always pick v2, even if QEMU reports v3 is emulatable
> > due to the v3 impl lacking MSI controller which we need for PCI-e
> 
> Our emulated GICv3 supports the ITS which has MSI support, so I'm not
> sure what forcing GICv2 is getting you here. Looking at the linked
> RHEL bugzilla bug, I suspect this is an out-of-date policy from before
> we added the ITS emulation in 2021 (it's present by default
> in virt-2.8 and later machine types). So that is something that
> libvirt should update I think.

It looks like it is virt-6.2 or later, and libvirt can probe it
by looking for existence of the arm-gicv3-its QOM type IIUC.

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|