KVM: arm64: Copy MIDR_EL1 into hyp VM when it is writable

KVM recently added a capability that allows userspace to override the
'implementation ID' registers presented to the VM. MIDR_EL1 is a special
example, where the hypervisor can directly set the value when read from
EL1 using VPIDR_EL2.

Copy the VM-wide value for MIDR_EL1 into the hyp VM for non-protected
guests when the capability is enabled so VPIDR_EL2 gets set up
correctly.

Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/kvmarm/ac594b9c-4bbb-46c8-9391-e7a68ce4de5b@sirena.org.uk/
Fixes: 3adaee7 ("KVM: arm64: Allow userspace to change the implementation ID registers")
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250305230825.484091-3-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/hyp/nvhe/pkvm.c

@@ -329,6 +329,10 @@ static void pkvm_init_features_from_host(struct pkvm_hyp_vm *hyp_vm, const struc
 		bitmap_copy(kvm->arch.vcpu_features,
 			    host_kvm->arch.vcpu_features,
 			    KVM_VCPU_MAX_FEATURES);
+
+		if (test_bit(KVM_ARCH_FLAG_WRITABLE_IMP_ID_REGS, &host_arch_flags))
+			hyp_vm->kvm.arch.midr_el1 = host_kvm->arch.midr_el1;
+
 		return;
 	}