KVM: arm64: Force ID_AA64PFR0_EL1.GIC=1 when exposing a virtual GICv3

Until now, we always let ID_AA64PFR0_EL1.GIC reflect the value
visible on the host, even if we were running a GICv2-enabled VM
on a GICv3+compat host.

That's fine, but we also now have the case of a host that does not
expose ID_AA64PFR0_EL1.GIC==1 despite having a vGIC. Yes, this is
confusing. Thank you M1.

Let's go back to first principles and expose ID_AA64PFR0_EL1.GIC=1
when a GICv3 is exposed to the guest. This also hides a GICv4.1
CPU interface from the guest which has no business knowing about
the v4.1 extension.
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211010150910.2911495-2-maz@kernel.org

arch/arm64/kvm/sys_regs.c

@@ -1075,6 +1075,11 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
 		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64PFR0_CSV2), (u64)vcpu->kvm->arch.pfr0_csv2);
 		val &= ~ARM64_FEATURE_MASK(ID_AA64PFR0_CSV3);
 		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64PFR0_CSV3), (u64)vcpu->kvm->arch.pfr0_csv3);
+		if (irqchip_in_kernel(vcpu->kvm) &&
+		    vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3) {
+			val &= ~ARM64_FEATURE_MASK(ID_AA64PFR0_GIC);
+			val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64PFR0_GIC), 1);
+		}
 		break;
 	case SYS_ID_AA64PFR1_EL1:
 		val &= ~ARM64_FEATURE_MASK(ID_AA64PFR1_MTE);