docs(spm): document s-el0 partition support

This patch adds a brief description of S-EL0 partition support in the
SPMC using ARMv8.1 FEAT_VHE.

Signed-off-by: Raghu Krishnamurthy <raghu.ncstate@gmail.com>
Change-Id: Ie079265476604f62d5f2a66684f01341000969d0

docs/components/secure-partition-manager.rst

@@ -919,6 +919,36 @@ streams.
    Fault handling, Performance Monitor Extensions, Event Handling, MPAM.
 -  No support for independent peripheral devices.
 
+S-EL0 Partition support
+=========================
+The SPMC (Hafnium) has limited capability to run S-EL0 FF-A partitions using
+FEAT_VHE (mandatory with ARMv8.1 in non-secure state, and in secure world
+with ARMv8.4 and FEAT_SEL2).
+
+S-EL0 partitions are useful for simple partitions that don't require full
+Trusted OS functionality. It is also useful to reduce jitter and cycle
+stealing from normal world since they are more lightweight than VMs.
+
+S-EL0 partitions are presented, loaded and initialized the same as S-EL1 VMs by
+the SPMC. They are differentiated primarily by the 'exception-level' property
+and the 'execution-ctx-count' property in the SP manifest. They are host apps
+under the single EL2&0 Stage-1 translation regime controlled by the SPMC and
+call into the SPMC through SVCs as opposed to HVCs and SMCs. These partitions
+can use FF-A defined services (FFA_MEM_PERM_*) to update or change permissions
+for memory regions.
+
+S-EL0 partitions are required by the FF-A specification to be UP endpoints,
+capable of migrating, and the SPMC enforces this requirement. The SPMC allows
+a S-EL0 partition to accept a direct message from secure world and normal world,
+and generate direct responses to them.
+
+Memory sharing between and with S-EL0 partitions is supported.
+Indirect messaging, Interrupt handling and Notifications are not supported with
+S-EL0 partitions and is work in progress, planned for future releases.
+All S-EL0 partitions must use AArch64. AArch32 S-EL0 partitions are not
+supported.
+
+
 References
 ==========