docs: document VM availability messages
Document VM availability direct message request/responses. Also document
that SP cannot respond to a framework direct request with a
non-framework direct response.
Change-Id: I05da447f5cdae23b250550a1a7bac014d82e2c0a
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/docs/secure-partition-manager/secure-partition-manager.rst b/docs/secure-partition-manager/secure-partition-manager.rst
index 7b32380..10025cd 100644
--- a/docs/secure-partition-manager/secure-partition-manager.rst
+++ b/docs/secure-partition-manager/secure-partition-manager.rst
@@ -853,6 +853,17 @@
- An SP cannot send a direct request to an Hypervisor or OS kernel.
- An Hypervisor or OS kernel can send a direct request to an SP.
- An SP can send a direct response to an Hypervisor or OS kernel.
+- An SP cannot reply to a framework direct request with a non-framework direct response.
+
+The hypervisor can inform SPs when a VM is created or destroyed by sending **VM
+availability messages** via the ``FFA_MSG_SEND_DIRECT_REQ`` ABI.
+
+A SP subscribes to receiving VM created and/or VM destroyed messages by
+specifying the ``vm-availability-messages`` field in its manifest (see
+`ffa-manifest-binding`_). The SPM will only forward messages to the SP if the SP
+is subscribed to the message kind. The SP must reply with the corresponding
+direct message response (via the ``FFA_MSG_SEND_DIRECT_RESP`` ABI) after it has
+handled the message.
FFA_MSG_SEND_DIRECT_REQ2/FFA_MSG_SEND_DIRECT_RESP2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~