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

[Mark Rutland]

On Fri, Oct 27, 2017 at 07:57:12AM +0100, Marc Zyngier wrote:
> On Thu, Oct 26 2017 at  4:48:39 pm BST, Mark Rutland <mark.rutland at arm.com> wrote:
> > On Fri, Oct 06, 2017 at 04:34:00PM +0100, Marc Zyngier wrote:

> >> @@ -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.
> 
> I'm afraid that won't be enough. If the CPU is brought up once we've
> already started a VM, we're screwed, as we cannot retroactively decide
> to drop GICv4 on the floor and nuke the guest. Or did you have something
> more radical in mind? Panic?

If you teach the arm64 cpufeature framework about this, it can abort bringing a
!GICv4 CPU online late.

It already handles onlining a !FP CPU if all boot CPUs had FP, for example.

See verify_local_cpu_features() in arch/arm64/kernel/cpufeature.c.

Thanks,
Mark.