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

On Mon, 24 Mar 2014, Julien Grall wrote:
> On 03/24/2014 12:03 PM, Julien Grall wrote:
> > On 03/24/2014 11:43 AM, Ian Campbell wrote:
> >> There is also the question of what guests use, which I suppose it mostly
> >> the same as dom0, but without the expectation that the guest kernel has
> >> to boot natively on a specific platform.
> >>
> >> I'm not sure what to suggest here, but I think for dom0 we can rely on
> >> "dom0 kernel should be usable as a native kernel on this platform" and
> >> we can punt for now on what to do about guests.
> > 
> > As GICv3 is 64-bits only, that would mean you can't boot 32bit dom0 on
> > this platform, right?
> Hmmm ... this question was stupid. GICv8 is ARMv8 specific not 64bits.
> Sorry for the noise.
> I think for the guest we have to support VGICv2 on GICv3 (if the
> hardware support it). The user might want to try his shiny OS which only
> support GICv2 on any hardware. It's easier than writing a support for GICv3.

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.

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?

Otherwise we could default to vgic_v2 if vcpus <= 8 and vgic_v3 if
vcpus > 8.

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