Re: [Xen-devel] [RFC PATCH v1 10/10] xen/arm: GICv3 device tree parsing

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 24 Mar 2014, Julien Grall wrote:
> Hi Stefano,
> 
> On 24/03/14 17:34, Stefano Stabellini wrote:
> > I think that for Dom0 we have to use vgic_v3, because it doesn't only
> > give you support for more vcpus but also MSI and MSI-X delivery.
> 
> Right.
> 
> > 
> > For DomUs it might be important to support vgic_v2 on gicv3 hardware,
> > however I wouldn't want to defer the decision to the user (i.e.
> > introduce yet another VM config option), if not for debugging.
> > 
> > Would it be possible to advirtise both gicv2 and gicv3 on device tree?
> > What would the guest kernel do in that case?
> 
> Linux will try to load both GICv2 and GICv3 drivers. In any case I don't think
> it's a solution because:
>       - you don't know how the kernel will react
>         - how will you choose which backend to use?
> 
> > Otherwise we could default to vgic_v2 if vcpus <= 8 and vgic_v3 if
> > vcpus > 8.
> 
> What about kernel which only support GICv3 and have less than 8 VCPUS?
> What about of device (MSI, ...) passthrough with this kind of kernel?
> 
> I think we need to have a VM config option in this specific case.

Having a VM config option is OK, but it should be the last resort for
people doing something very uncommon.

GICv3 support in Linux and distros is probably going to be widespread
soon, so we might not have to worry about it though.

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