KVM: arm64: Zero ID_AA64PFR0_EL1.GIC when no GICv3 is presented to the guest

In order to be consistent, we shouldn't advertise a GICv3 when none
is actually usable by the guest.

Wipe the feature when these conditions apply, and allow the field
to be written from userspace.

This now allows us to rewrite the kvm_has_gicv3 helper() in terms
of kvm_has_feat(), given that it is always evaluated at runtime.
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240827152517.3909653-6-maz@kernel.orgSigned-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/sys_regs.c

@@ -2365,7 +2365,6 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 		   ID_AA64PFR0_EL1_MPAM |
 		   ID_AA64PFR0_EL1_SVE |
 		   ID_AA64PFR0_EL1_RAS |
-		   ID_AA64PFR0_EL1_GIC |
 		   ID_AA64PFR0_EL1_AdvSIMD |
 		   ID_AA64PFR0_EL1_FP), },
 	ID_SANITISED(ID_AA64PFR1_EL1),
@@ -4634,6 +4633,13 @@ int kvm_finalize_sys_regs(struct kvm_vcpu *vcpu)
 
 	guard(mutex)(&kvm->arch.config_lock);
 
+	if (!(static_branch_unlikely(&kvm_vgic_global_state.gicv3_cpuif) &&
+	      irqchip_in_kernel(kvm) &&
+	      kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3)) {
+		kvm->arch.id_regs[IDREG_IDX(SYS_ID_AA64PFR0_EL1)] &= ~ID_AA64PFR0_EL1_GIC_MASK;
+		kvm->arch.id_regs[IDREG_IDX(SYS_ID_PFR1_EL1)] &= ~ID_PFR1_EL1_GIC_MASK;
+	}
+
 	if (vcpu_has_nv(vcpu)) {
 		int ret = kvm_init_nv_sysregs(kvm);
 		if (ret)

arch/arm64/kvm/vgic/vgic.h

@@ -350,9 +350,7 @@ void vcpu_set_ich_hcr(struct kvm_vcpu *vcpu);
 
 static inline bool kvm_has_gicv3(struct kvm *kvm)
 {
-	return (static_branch_unlikely(&kvm_vgic_global_state.gicv3_cpuif) &&
-		irqchip_in_kernel(kvm) &&
-		kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3);
+	return kvm_has_feat(kvm, ID_AA64PFR0_EL1, GIC, IMP);
 }
 
 #endif