KVM: s390: Allow userspace to limit guest memory size

With commit c6c956b80b ("KVM: s390/mm: support gmap page tables with less
than 5 levels") we are able to define a limit for the guest memory size.

As we round up the guest size in respect to the levels of page tables
we get to guest limits of: 2048 MB, 4096 GB, 8192 TB and 16384 PB.
We currently limit the guest size to 16 TB, which means we end up
creating a page table structure supporting guest sizes up to 8192 TB.

This patch introduces an interface that allows userspace to tune
this limit. This may bring performance improvements for small guests.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

Documentation/virtual/kvm/devices/vm.txt

@@ -24,3 +24,17 @@ Returns: 0
 
 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.
+
+1.3. ATTRIBUTE KVM_S390_VM_MEM_LIMIT_SIZE
+Parameters: in attr->addr the address for the new limit of guest memory
+Returns: -EFAULT if the given address is not accessible
+         -EINVAL if the virtual machine is of type UCONTROL
+         -E2BIG if the given guest memory is to big for that machine
+         -EBUSY if a vcpu is already defined
+         -ENOMEM if not enough memory is available for a new shadow guest mapping
+          0 otherwise
+
+Allows userspace to query the actual limit and set a new limit for
+the maximum guest memory size. The limit will be rounded up to
+2048 MB, 4096 GB, 8192 TB respectively, as this limit is governed by
+the number of page table levels.