OvmfPkg/XenBusDxe: Add Event Channel into XenBus protocol.

This patch adds three event channel related functions:
- EventChannelAllocate: Allocate an event channel port that can be bind
  from a specified domain.
- EventChannelNotify: Send an event to the remote end of a channel.
- EventChannelClose: Close a local event channel port.

Change in V3:
- eventchannel, update protocol to return error code.
- expand patch description
- Add comments in the XenBus Protocol header.

Change in V2:
- coding style
- adding comment to functions
- Rename Xenbus to XenBus.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16271 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/XenBusDxe/EventChannel.h

@@ -33,4 +33,56 @@ XenEventChannelNotify (
   IN evtchn_port_t Port
   );
 
+/*
+ * XenBus protocol
+ */
+
+/**
+  Allocate a port that can be bind from domain DomainId.
+
+  @param This       A pointer to the XENBUS_PROTOCOL.
+  @param DomainId   The domain ID that can bind the newly allocated port.
+  @param Port       A pointer to a evtchn_port_t that will contain the newly
+                    allocated port.
+
+  @retval UINT32    The return value from the hypercall, 0 if success.
+**/
+UINT32
+EFIAPI
+XenBusEventChannelAllocate (
+  IN  XENBUS_PROTOCOL *This,
+  IN  domid_t         DomainId,
+  OUT evtchn_port_t   *Port
+  );
+
+/**
+  Send an event to the remote end of the channel whose local endpoint is Port.
+
+  @param This       A pointer to the XENBUS_PROTOCOL.
+  @param Port       Local port to the the event from.
+
+  @retval UINT32    The return value from the hypercall, 0 if success.
+**/
+UINT32
+EFIAPI
+XenBusEventChannelNotify (
+  IN XENBUS_PROTOCOL *This,
+  IN evtchn_port_t   Port
+  );
+
+/**
+  Close a local event channel Port.
+
+  @param This       A pointer to the XENBUS_PROTOCOL.
+  @param Port       The event channel to close.
+
+  @retval UINT32    The return value from the hypercall, 0 if success.
+**/
+UINT32
+EFIAPI
+XenBusEventChannelClose (
+  IN XENBUS_PROTOCOL *This,
+  IN evtchn_port_t   Port
+  );
+
 #endif