KVM: arm64: gic-v5: Sanitize ID_AA64PFR2_EL1.GCIE

Add in a sanitization function for ID_AA64PFR2_EL1, preserving the
already-present behaviour for the FPMR, MTEFAR, and MTESTOREONLY
fields. Add sanitisation for the GCIE field, which is set to IMP if
the host supports a GICv5 guest and NI, otherwise.

Extend the sanitisation that takes place in kvm_vgic_create() to zero
the ID_AA64PFR2.GCIE field when a non-GICv5 GIC is created. More
importantly, move this sanitisation to a separate function,
kvm_vgic_finalize_sysregs(), and call it from kvm_finalize_sys_regs().

We are required to finalize the GIC and GCIE fields a second time in
kvm_finalize_sys_regs() due to how QEMU blindly reads out then
verbatim restores the system register state. This avoids the issue
where both the GCIE and GIC features are marked as present (an
architecturally invalid combination), and hence guests fall over. See
the comment in kvm_finalize_sys_regs() for more details.

Overall, the following happens:

* Before an irqchip is created, FEAT_GCIE is presented if the host
  supports GICv5-based guests.
* Once an irqchip is created, all other supported irqchips are hidden
  from the guest; system register state reflects the guest's irqchip.
* Userspace is allowed to set invalid irqchip feature combinations in
  the system registers, but...
* ...invalid combinations are removed a second time prior to the first
  run of the guest, and things hopefully just work.

All of this extra work is required to make sure that "legacy" GICv3
guests based on QEMU transparently work on compatible GICv5 hosts
without modification.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20260319154937.3619520-13-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

include/kvm/arm_vgic.h

@@ -485,6 +485,7 @@ int kvm_vgic_create(struct kvm *kvm, u32 type);
 void kvm_vgic_destroy(struct kvm *kvm);
 void kvm_vgic_vcpu_destroy(struct kvm_vcpu *vcpu);
 int kvm_vgic_map_resources(struct kvm *kvm);
+void kvm_vgic_finalize_idregs(struct kvm *kvm);
 int kvm_vgic_hyp_init(void);
 void kvm_vgic_init_cpu_hardware(void);