KVM: arm64: nv: Allocate VNCR page when required

If running a NV guest on an ARMv8.4-NV capable system, let's
allocate an additional page that will be used by the hypervisor
to fulfill system register accesses.

Reviewed-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250514103501.2225951-3-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Marc Zyngier

arch/arm64/kvm/nested.c

@@ -55,6 +55,13 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
 	    !cpus_have_final_cap(ARM64_HAS_HCR_NV1))
 		return -EINVAL;
 
+	if (!vcpu->arch.ctxt.vncr_array)
+		vcpu->arch.ctxt.vncr_array = (u64 *)__get_free_page(GFP_KERNEL_ACCOUNT |
+								    __GFP_ZERO);
+
+	if (!vcpu->arch.ctxt.vncr_array)
+		return -ENOMEM;
+
 	/*
 	 * Let's treat memory allocation failures as benign: If we fail to
 	 * allocate anything, return an error and keep the allocated array
@@ -85,6 +92,9 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
 		for (int i = kvm->arch.nested_mmus_size; i < num_mmus; i++)
 			kvm_free_stage2_pgd(&kvm->arch.nested_mmus[i]);
 
+		free_page((unsigned long)vcpu->arch.ctxt.vncr_array);
+		vcpu->arch.ctxt.vncr_array = NULL;
+
 		return ret;
 	}
 

arch/arm64/kvm/reset.c

@@ -158,6 +158,7 @@ void kvm_arm_vcpu_destroy(struct kvm_vcpu *vcpu)
 	if (sve_state)
 		kvm_unshare_hyp(sve_state, sve_state + vcpu_sve_state_size(vcpu));
 	kfree(sve_state);
+	free_page((unsigned long)vcpu->arch.ctxt.vncr_array);
 	kfree(vcpu->arch.ccsidr);
 }