KVM: s390: provide CMMA attributes only if available

Let's not provide the device attribute for cmma enabling and clearing
if the hardware doesn't support it.

This also helps getting rid of the undocumented return value "-EINVAL"
in case CMMA is not available when trying to enable it.

Also properly document the meaning of -EINVAL for CMMA clearing.

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

Documentation/virtual/kvm/api.txt

@@ -2520,6 +2520,7 @@ Parameters: struct kvm_device_attr
 Returns: 0 on success, -1 on error
 Errors:
   ENXIO:  The group or attribute is unknown/unsupported for this device
+          or hardware support is missing.
   EPERM:  The attribute cannot (currently) be accessed this way
           (e.g. read-only attribute, or attribute that only makes
           sense when the device is in a different state)
@@ -2547,6 +2548,7 @@ Parameters: struct kvm_device_attr
 Returns: 0 on success, -1 on error
 Errors:
   ENXIO:  The group or attribute is unknown/unsupported for this device
+          or hardware support is missing.
 
 Tests whether a device supports a particular attribute.  A successful
 return indicates the attribute is implemented.  It does not necessarily

Documentation/virtual/kvm/devices/vm.txt

@@ -20,7 +20,8 @@ Enables Collaborative Memory Management Assist (CMMA) for the virtual machine.
 
 1.2. ATTRIBUTE: KVM_S390_VM_MEM_CLR_CMMA
 Parameters: none
-Returns: 0
+Returns: -EINVAL if CMMA was not enabled
+         0 otherwise
 
 Clear the CMMA status for all guest pages, so any pages the guest marked
 as unused are again used any may not be reclaimed by the host.

arch/s390/kvm/kvm-s390.c

@@ -485,7 +485,7 @@ static int kvm_s390_set_mem_control(struct kvm *kvm, struct kvm_device_attr *att
 	unsigned int idx;
 	switch (attr->attr) {
 	case KVM_S390_VM_MEM_ENABLE_CMMA:
-		ret = -EINVAL;
+		ret = -ENXIO;
 		if (!sclp.has_cmma)
 			break;
 
@@ -499,6 +499,9 @@ static int kvm_s390_set_mem_control(struct kvm *kvm, struct kvm_device_attr *att
 		mutex_unlock(&kvm->lock);
 		break;
 	case KVM_S390_VM_MEM_CLR_CMMA:
+		ret = -ENXIO;
+		if (!sclp.has_cmma)
+			break;
 		ret = -EINVAL;
 		if (!kvm->arch.use_cmma)
 			break;
@@ -964,6 +967,8 @@ static int kvm_s390_vm_has_attr(struct kvm *kvm, struct kvm_device_attr *attr)
 		switch (attr->attr) {
 		case KVM_S390_VM_MEM_ENABLE_CMMA:
 		case KVM_S390_VM_MEM_CLR_CMMA:
+			ret = sclp.has_cmma ? 0 : -ENXIO;
+			break;
 		case KVM_S390_VM_MEM_LIMIT_SIZE:
 			ret = 0;
 			break;