OvmfPkg/XenBusDxe: Add Event Channel Notify.

This first function is used to notify the other side that there is
something to do. The other side is another Xen domain.

Change in V4:
- Replace the license by the commonly used file header text.

Change in V3:
- Return error code from hypercall instead of ASSERT for
  XenEventChannelNotify
- moving event_channel.h to this patch.

Change in V2:
- file header
- coding style
- adding comment to functions
- Licenses

License: This patch adds event_channel.h which is under MIT licence.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16265 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/XenBusDxe/EventChannel.c

@@ -0,0 +1,33 @@
+/** @file
+  Event Channel function implementation.
+
+  Event channel are use to notify of an event that happend in a shared
+  structure for example.
+
+  Copyright (C) 2014, Citrix Ltd.
+
+  This program and the accompanying materials
+  are licensed and made available under the terms and conditions of the BSD License
+  which accompanies this distribution.  The full text of the license may be found at
+  http://opensource.org/licenses/bsd-license.php
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+
+**/
+#include "EventChannel.h"
+#include "XenHypercall.h"
+
+UINT32
+XenEventChannelNotify (
+  IN XENBUS_DEVICE *Dev,
+  IN evtchn_port_t Port
+  )
+{
+  INTN ReturnCode;
+  evtchn_send_t Send;
+
+  Send.port = Port;
+  ReturnCode = XenHypercallEventChannelOp (Dev, EVTCHNOP_send, &Send);
+  return ReturnCode;
+}