[PATCH v4 25/26] KVM: arm/arm64: GICv4: Prevent heterogenous systems from using GICv4

[mark.rutland@arm.com (Mark Rutland)]

Hi,

On Fri, Oct 06, 2017 at 04:34:00PM +0100, Marc Zyngier wrote:
> The GICv4 architecture doesn't prevent CPUs implementing GICv4 to
> cohabit with CPUs limited to GICv3 in the same system.
> 
> This is mad (the scheduler would have to be made aware of the v4
> capability), and we're certainly not going to support this any
> time soon. So let's check that all online CPUs are GICv4 capable,
> and disable the functionnality if not.

I just spotted this because of Christoffer's review; apologies for the late
drive-by comment...

> @@ -485,8 +495,21 @@ int vgic_v3_probe(const struct gic_kvm_info *info)
>  	kvm_vgic_global_state.can_emulate_gicv2 = false;
>  	kvm_vgic_global_state.ich_vtr_el2 = ich_vtr_el2;
>  
> -	/* GICv4 support? */
> +	/*
> +	 * GICv4 support? We need to check on all CPUs in case of some
> +	 * extremely creative form of big-little brain damage...
> +	 */
>  	if (info->has_v4) {
> +		int cpu;
> +
> +		for_each_online_cpu(cpu) {
> +			bool enable;
> +
> +			smp_call_function_single(cpu, vgic_check_v4_cpuif,
> +						 &enable, 1);
> +			gicv4_enable = gicv4_enable && enable;
> +		}

With maxcpus=N on the command line, CPUs can be brought online late, so we
might need this in a hotplug callback (and/or in the arm64 cpufeature
framework) to handle that case.

Thanks,
Mark.