This patch introduces a new feature to support the signaling between
two domains in dom0less system.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
---
v2 changes:
- switch to the one-subnode-per-evtchn under xen,domain" compatible node.
- Add more detail about event-channel 
---
 docs/designs/dom0less-evtchn.md | 126 ++++++++++++++++++++++++++++++++
 1 file changed, 126 insertions(+)
 create mode 100644 docs/designs/dom0less-evtchn.md

diff --git a/docs/designs/dom0less-evtchn.md b/docs/designs/dom0less-evtchn.md
new file mode 100644
index 0000000000..62ec8a4009
--- /dev/null
+++ b/docs/designs/dom0less-evtchn.md
@@ -0,0 +1,126 @@
+# Signaling support between two domUs on dom0less system
+
+## Current state: Draft version
+
+## Proposer(s): Rahul Singh, Bertrand Marquis
+
+## Problem Statement:
+
+Dom0less guests would benefit from a statically-defined memory sharing and
+signally system for communication. One that would be immediately available at
+boot without any need for dynamic configurations.
+
+In embedded a great variety of guest operating system kernels exist, many of
+which don't have support for xenstore, grant table, or other complex drivers.
+Some of them are small kernel-space applications (often called "baremetal",
+not to be confused with the term "baremetal" used in the data center which
+means "without hypervisors") or RTOSes. Additionally, for safety reasons, users
+often need to be able to configure the full system statically so that it can
+be verified statically.
+
+Event channels are very simple and can be added even to baremetal applications.
+This proposal introduces a way to define them statically to make them suitable
+for dom0less embedded deployments.
+
+## Proposal:
+
+Event channels are the basic primitive provided by Xen for event notifications.
+An event channel is a logical connection between 2 domains (more specifically
+between dom1,port1, and dom2,port2). Each event has a pending and a masked bit.
+The pending bit indicates the event has been raised. The masked bit is used by
+the domain to prevent the delivery of that specific event. Xen only performs a
+0 → 1 transition on the pending bits and does not touch the mask bit. The
+domain may toggle masked bits in the masked bit field and should clear the
+pending bit when an event has been processed
+
+Events are received by a domain via an interrupt from Xen to the domain,
+indicating when an event arrives (setting the bit). Further notifications are
+blocked until the bit is cleared again. Events are delivered asynchronously to
+a domain and are enqueued when the domain is not running.
+More information about FIFO based event channel can be found at:
+https://xenbits.xen.org/people/dvrabel/event-channels-H.pdf
+
+The event channel communication will be established statically between two
+domains (dom0 and domU also) before unpausing the domains after domain creation.
+Event channel connection information between domains will be passed to XEN via
+the device tree node. The event channel will be created and established
+beforehand in XEN before the domain started. The domain doesn’t need to do any
+operation to establish a connection. Domain only needs hypercall
+EVTCHNOP_send(local port) to send notifications to the remote guest.
+
+There is no need to describe the static event channel info in the domU device
+tree. Static event channels are only useful in fully static configurations,
+and in those configurations the domU device tree dynamically generated by Xen
+is not needed.
+
+Under the "xen,domain" compatible node, there need to be sub-nodes with
+compatible "xen,evtchn" that describe the event channel connection between two
+domains(dom0 and domU also).
+
+The event channel sub-node has the following properties:
+
+- compatible
+
+    "xen,evtchn"
+
+- xen,evtchn
+
+    The property is tuples of two numbers
+    (local-evtchn link-to-foreign-evtchn) where:
+
+    local-evtchn is an integer value that will be used to allocate local port
+    for a domain to send and receive event notifications to/from the remote
+    domain.
+
+    link-to-foreign-evtchn is a single phandle to a remote evtchn to which
+    local-evtchn will be connected.
+
+
+Example:
+
+    chosen {
+        ....
+
+        domU1: domU1 {
+            compatible = "xen,domain";
+
+            /* one sub-node per local event channel */
+            ec1: evtchn@1 {
+                compatible = "xen,evtchn-v1";
+                /* local-evtchn link-to-foreign-evtchn */
+                xen,evtchn = <0xa &ec3>;
+            };
+
+            ec2: evtchn@2 {
+                compatible = "xen,evtchn-v1";
+                xen,evtchn = <0xc &ec4>;
+            };
+            ....
+        };
+
+        domU2: domU2 {
+            compatible = "xen,domain";
+
+            /* one sub-node per local event channel */
+            ec3: evtchn@3 {
+                compatible = "xen,evtchn-v1";
+                /* local-evtchn link-to-foreign-evtchn */
+                xen,evtchn = <0xb &ec1>;
+            };
+
+            ec4: evtchn@4 {
+                compatible = "xen,evtchn-v1";
+                xen,evtchn = <0xd &ec2>;
+            };
+            ....
+        };
+    };
+
+In above example two event channel comunication will be established between
+domU1 and domU2.
+
+    domU1 (port 0xa) <-----------------> domU2 (port 0xb)
+    domU1 (port 0xc) <-----------------> domU2 (port 0xd)
+
+domU1 and domU2 can send the signal to remote domain via hypercall
+EVTCHNOP_send(.) on local port.
-- 
2.25.1