Let's document our new diag500 subfunction that can be implemented by
userspace.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 Documentation/virt/kvm/s390/s390-diag.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/virt/kvm/s390/s390-diag.rst b/Documentation/virt/kvm/s390/s390-diag.rst
index d9b7c6cbc99e..c69c6e0fa71b 100644
--- a/Documentation/virt/kvm/s390/s390-diag.rst
+++ b/Documentation/virt/kvm/s390/s390-diag.rst
@@ -77,6 +77,23 @@ Subcode 3 - virtio-ccw notification
 
     See also the virtio standard for a discussion of this hypercall.
 
+Subcode 4 - storage-limit
+    Handled by userspace.
+
+    After completion of the DIAGNOSE call, general register 2 will
+    contain the storage limit: the maximum physical address that might be
+    used for storage throughout the lifetime of the VM.
+
+    The storage limit does not indicate currently usable storage, it may
+    include holes, standby storage and areas reserved for other means, such
+    as memory hotplug or virtio-mem devices. Other interfaces for detecting
+    actually usable storage, such as SCLP, must be used in conjunction with
+    this subfunction.
+
+    Note that the storage limit can be larger, but never smaller than the
+    maximum storage address indicated by SCLP via the "maximum storage
+    increment" and the "increment size".
+
 
 DIAGNOSE function code 'X'501 - KVM breakpoint
 ----------------------------------------------
-- 
2.46.1