dataplane: add event loop

Outside the safety of the global mutex we need to poll on file
descriptors.  I found epoll(2) is a convenient way to do that, although
other options could replace this module in the future (such as an
AioContext-based loop or glib's GMainLoop).

One important feature of this small event loop implementation is that
the loop can be terminated in a thread-safe way.  This allows QEMU to
stop the data plane thread cleanly.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/dataplane/event-poll.h

@@ -0,0 +1,40 @@
+/*
+ * Event loop with file descriptor polling
+ *
+ * Copyright 2012 IBM, Corp.
+ * Copyright 2012 Red Hat, Inc. and/or its affiliates
+ *
+ * Authors:
+ *   Stefan Hajnoczi <stefanha@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#ifndef EVENT_POLL_H
+#define EVENT_POLL_H
+
+#include "qemu/event_notifier.h"
+
+typedef struct EventHandler EventHandler;
+typedef void EventCallback(EventHandler *handler);
+struct EventHandler {
+    EventNotifier *notifier;        /* eventfd */
+    EventCallback *callback;        /* callback function */
+};
+
+typedef struct {
+    int epoll_fd;                   /* epoll(2) file descriptor */
+    EventNotifier stop_notifier;    /* stop poll notifier */
+    EventHandler stop_handler;      /* stop poll handler */
+} EventPoll;
+
+void event_poll_add(EventPoll *poll, EventHandler *handler,
+                    EventNotifier *notifier, EventCallback *callback);
+void event_poll_init(EventPoll *poll);
+void event_poll_cleanup(EventPoll *poll);
+void event_poll(EventPoll *poll);
+void event_poll_notify(EventPoll *poll);
+
+#endif /* EVENT_POLL_H */