Documentation: s390-diag.rst: Document diag500(STORAGE LIMIT) subfunction

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

Signed-off-by: David Hildenbrand <[e-mail]>
Reviewed-by: Heiko Carstens <[e-mail]>
Reviewed-by: Eric Farman <[e-mail]>
Tested-by: Sumanth Korikkar <[e-mail]>
Acked-by: Christian Borntraeger <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Heiko Carstens <[e-mail]>

Documentation/virt/kvm/s390/s390-diag.rst

···
 
     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
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