doc: add hypercall ABI in the HLD

The application binary interface (ABI) is an important aspect of
hypercalls. But unfortunately it is not yet defined anywhere in the current
HLD.

This patch adds the current ABI specification to the hypercall section of
HLD.

Signed-off-by: Junjie Mao <junjie.mao@intel.com>

doc/developer-guides/hld/hv-hypercall.rst

@@ -9,6 +9,29 @@ is from the hypervisor to the guest VM. The hypervisor supports
 hypercall APIs for VM management, I/O request distribution, interrupt injection,
 PCI assignment, guest memory mapping, power management, and secure world switch.
 
+The application binary interface (ABI) of ACRN hypercalls is defined as follows.
+
+- A guest VM executes the ``vmcall`` instruction to trigger a hypercall.
+
+- Input parameters of a hypercall includes:
+
+  - An hypercall ID in register ``R8``, which specifies the kind of service
+    requested by the guest VM.
+
+  - The first parameter in register ``RDI`` and the second in register
+    ``RSI``. The semantics of those two parameters vary among different kinds of
+    hypercalls and are defined in the hypercall APIs reference. For hypercalls
+    requesting operations on a specific VM, the first parameter is typically the
+    ID of that VM.
+
+- The register ``RAX`` contains the return value of the hypercall after a guest
+  VM executes the ``vmcall`` instruction, unless the ``vmcall`` instruction
+  triggers an exception. Other general purpose registers are not modified by a
+  hypercall.
+
+- In case a hypercall parameter is defined as a pointer to a data structure,
+  fields in that structure can be either input, output or inout.
+
 There are some restrictions for hypercall and upcall:
 
 #. Only specific VMs (the Service VM and the VM with Trusty enabled)