KVM: x86/xen: Fix return code when clearing vcpu_info and vcpu_time_info

When clearing the per-vCPU shared regions, set the return value to zero
to indicate success. This was causing spurious errors to be returned to
userspace on soft reset.

Also add a paranoid BUILD_BUG_ON() for compat structure compatibility.

Fixes: 0c165b3c ("KVM: x86/xen: Allow reset of Xen attributes")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20210301125309.874953-1-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/xen.c

@@ -187,9 +187,12 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		/* No compat necessary here. */
 		BUILD_BUG_ON(sizeof(struct vcpu_info) !=
 			     sizeof(struct compat_vcpu_info));
+		BUILD_BUG_ON(offsetof(struct vcpu_info, time) !=
+			     offsetof(struct compat_vcpu_info, time));
 
 		if (data->u.gpa == GPA_INVALID) {
 			vcpu->arch.xen.vcpu_info_set = false;
+			r = 0;
 			break;
 		}
 
@@ -206,6 +209,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 	case KVM_XEN_VCPU_ATTR_TYPE_VCPU_TIME_INFO:
 		if (data->u.gpa == GPA_INVALID) {
 			vcpu->arch.xen.vcpu_time_info_set = false;
+			r = 0;
 			break;
 		}